Method for production of hydrophilic resin

ABSTRACT

In the production of an acrylate polymer by the steps of preparing a water-soluble unsaturated monomer having a ratio of neutralization in the range of 30 to 100 mol % and containing 50 to 100 mol % of an acrylate and then polymerizing the monomer, a method for the production of a hydrophilic resin which comprises using a water-soluble unsaturated monomer having a β-hydroxy propionic acid (salt) content of not more than 1,000 ppm. The hydrophilic resin obtained by this method exhibits excellent physical properties and has only a small residual monomer content. Further, a residual monomer content neither occurs nor increases in any using conditions.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method for the production of ahydrophilic resin having an acrylate as a main component thereof. Moreparticularly it relates to a method for the production of a hydrophilicresin which has only a small residual monomer content and showsvirtually no increase in the residual monomer content under varyingconditions of use.

[0003] The hydrophilic resin according to this invention can be producedeasily and inexpensively and is excellent in quality and safety and,therefore, can be used as an absorbent resin and water-soluble resin ina wide range of applications.

[0004] 2. Description of the Prior Art

[0005] The hydrophilic resins can be generally classified by theirsolubility in water into roughly two types, water-soluble resins andabsorbent resins.

[0006] Water-soluble resins are hydrophilic resins of the type whichdissolve in water and are used, for example, as water treatment gradeflocculants, oil drilling additives, food additives, and viscosityenhancers.

[0007] The water-soluble resins which are known to the art include, forexample, polysodium acrylate (JP-B-48-42,466 and JP-B-42-9,656),polyacrylic acid and polyacrylamide (JP-A-54-145,782 andJP-A-57-18,652), polymers of 2-acrylamide-2-methylpropane sulfonic acid(JP-A-2-173,108), partial hydrolyzate of polyacrylamide(JP-A-52-137,483), acrylic acid-acrylamide copolymer (JP-A-59-15,417),(meth)acrylic acid-itaconic acid copolymer (JP-A-58-91,709), andpolyvinyl alcohol.

[0008] Absorbent resins are water-insoluble hydrophilic resins of thetype which absorb water and consequently undergo gelation and are widelyused in the fields of agriculture and forestry and in the field of civilengineering as well as in the field of hygienic materials such asdisposable diapers and sanitary napkins.

[0009] The absorbent resins which have been heretofore known include,for example, partially neutralized cross-linked polyacrylic acid(JP-A-55-84,304, JP-A-55-108,407, and JP-A-55-133,413), hydrolyzate ofstarch-acrylonitrile graft polymer (JP-A-46-43,995), neutralizedstarch-acrylic acid graft polymer (JP-A-51-125,468), saponified vinylacetate-acrylic ester copolymer (JP-A-52-14,689), hydrolyzate ofacrylonitrile copolymer or acrylamide copolymer (JP-A-53-15,959) orcross-linked derivatives thereof, and cross-linked cationic monomer(JP-A-58-154,709 and JP-A-58-154, 710).

[0010] Numerous compounds have been proposed as monomers for theproduction of these hydrophilic resins. From the viewpoint of thequality of the product and the cost of production, a partially orcompletely neutralized acrylate (hereinafter referred to as “acrylate”)is predominately used today. The acrylate type polymers which haveacrylates as the main component of their monomers are produced today inlarge amounts for both absorbent resins and water-soluble resins andhave been finding extensive utility in the fields of hygienic materialsand foodstuffs.

[0011] By the current technical standard, it is normal that the acrylatetype polymers which are, in wide use generally contain such a residualmonomer as unaltered acrylic acid (or a salt thereof) in a concentrationin the range of 500 to 3,000 ppm. Thus, the desirability of decreasingthe residual monomer content in the polymers has been findingenthusiastic recognition.

[0012] In these hydrophilic resins, particularly the absorbent resinsused in sanitary materials, a lower the residual monomer content isrequired. In recent years, the prevailing demand is to lower theresidual monomer content to below 100 ppm. This decrease of the residualmonomer content is particularly difficult to attain in the absorbentresins among the hydrophilic resins because the absorbent resins arehydrophilic resins of the type having a cross-linked structure and,therefore, more often than not have a neutral pH value.

[0013] The absorbent resins having a cross-linked structure are noteasily polymerized uniformly as compared with water-soluble resins. Whenabsorbent resins fresh from polymerization are to be mixed with anadditive to decrease the residual monomer content or with an organicsolvent, a uniform mixture is not easily obtained because of thecross-linked structure of the polymers. Thus, a decrease of the residualmonomer content in the absorbent resins has been extremely difficult toattain. Moreover, since acrylates are such that their polymerizationvelocities are lowered proportionately as their pH values approachneutrality, a decrease of the residual monomer content as a consequenceof polymerization has been extremely difficult to attain in neutralacrylate type absorbent resins.

[0014] Heretofore, in the field of macromolecular flocculants, forexample, there has been an attempt at decreasing the residual monomercontent in hydrophilic resins. Even now, numerous hydro-philic resinsmentioned above as well as acrylate type polymers and absorbent resinsare still the subjects of a study in a search for measures of decreasingthe residual monomer content.

[0015] The techniques known to the art are broadly divided into thefollowing six types (a) to (f):

[0016] (a) Methods for lowering the residual monomer content byincreasing the polymerization ratio of the polymer itself.

[0017] The methods of this type include, for example, increase of theamount of polymerization initiator and the use of a composite initiator(JP-A-50-96,689), elevation of the reaction temperature, an increase inthe polymerization concentration, lengthening the polymerization time,and the specification of aging conditions (JP-A-53-145,895), thetwo-stage addition of a polymerization initiator (JP-A-56-72,005), theexposure of a resin fresh from polymerization to radiation(JP-A-63-43,930), and the irradiation of a resin fresh frompolymerization with ultraviolet light (JP-A-62-260,906).

[0018] (b) Methods for converting the residual monomer in the polymerinto an extraneous derivative by use of an additive.

[0019] The methods of this type include, for example, the subsequentaddition of a primary or secondary amine (JP-A-50-40,649), thesubsequent addition of sulfur dioxide (U.S. Pat. No. 3,780,006), and thesubsequent addition of an alkali metabisulfite (U.S. Pat. No.4,306,955).

[0020] (c) Methods for extracting the residual monomer from the polymer.

[0021] The methods of this type include, for example, the extraction bythe use of a hydrophilic organic solvent (U.S. Pat. No. 4,794,116) andthe supercritical extraction by the use of carbon dioxide.

[0022] (d) Methods for treating the residual monomer with amicro-organism capable of decomposing the residual monomer

[0023] The methods of this type include, for example, decomposition ofresidual acrylamide with a microorganism (U.S. Pat. No. 4,742,114).

[0024] (e) Methods for volatilizing the residual monomer at elevatedtemperatures

[0025] The methods of this type include, for example, volatilization ofresidual acrylonitrile at an elevated temperature (JP-A-54-119,588).

[0026] The methods of (a), however, are actually such that since theireffects in lowering the residual monomer content are not sufficient, theresidual monomer generally persists in a concentration of at least 0.03%and the self-crosslinking occurs and basic molecular weight of thehydrophilic resin are necessarily degraded by the harsh conditionsduring polymerization and the aftertreatment possibly to the extent ofincreasing the water-soluble content of the absorbent resin, loweringthe gel strength, and impairing the physical properties of the resultinghydrophilic resin.

[0027] Further, the two-stage addition of a polymerization initiator andthe use of a large amount of initiator increases the possibility of thepolymerization initiator persisting in the produced resin andconsequently jeopardizing the safety of the produced polymer.

[0028] The methods of (b) and (c) are purportedly capable of loweringthe residual monomer content to below 0.03%. However in (b), in additionto complexed process, an additive used and the adduct formed of theadditive with the residual monomer and in (c) the organic solvent suchas methanol used for the extraction of residual monomer, never fail topersist in the hydrophilic resin.

[0029] The effect of the method of (c) in lowering the residual monomercontent in the resin, however, is limited because acrylates are notdissolved in such organic solvents as methanol.

[0030] The method of (d) is not easily carried out on a commercial scalebecause of the use of a microorganism. Moreover, the use of themicroorganism itself proves to be undesirable from the standpoint ofsafety.

[0031] The method of (e) is observed at times to impair various physicalproperties by the elevated temperatures. Moreover, since the acrylatefails to volatilize even at elevated temperatures, this method canhardly be expected to attain an effective decrease of the residualmonomer content.

[0032] More recently, (f) the efforts directed to the reduction of theresidual monomer content have revealed in the water-soluble unsaturatedmonomer as a matter deserving due attention before polymerization. Themethod embodying this knowledge has also been known to the art.

[0033] The methods of this type already known to the art includepolymerization effected by the use of an acrylate obtained by a specificmethod of neutralization (EP-A-0372706) and polymeri-zation attained bythe use of a monomer having a small heavy metal content (JP-A-3-31,306),for example.

[0034] The methods of the type of (f), however, are not sufficientlyeffective in lowering the residual monomer content.

[0035] The methods of the various types cited above, however, areactually such that they not only fail to produce the required effect butalso necessitate a complicated process and involve a sacrifice inproductivity and physical properties and a large addition to the cost ofproduction. Moreover, these methods only bring about an apparentdecrease in the residual monomer content in the acrylate type polymerand are totally incapable of repressing the increase of the residualmonomer content which occurs subsequent to the polymerization asdescribed hereinbelow.

[0036] We have found that in the acrylate polymer which is obtained bythe conventional method, an unaltered monomer persists in such highconcentrations as to fall in- the approximate range of some tens of ppmto some thousands of ppm in addition to the residual monomer of the typegenerally known in the art. We have also found that even when theresidual monomer content of the acrylate polymer is decreased apparentlyto a level of some hundreds of ppm, the residual monomer actuallyincreases proportionately over a period of time. This increase over aperiod of time is particularly conspicuous when the polymer is heated.

[0037] When the acrylate polymer having an apparently small residualmonomer content is required to undergo further heating or when it isused in an agronomic field for a long time or exposed to an elevatedtemperature such as, for example, hot water, it is natural to concludethat this use of the polymer is undesirable from the viewpoint of safetybecause the residual monomer content is increased under such conditionsof use.

[0038] Concerning the production of the acrylate polymer, manytechniques have been known to cross-link the surface region of theacrylate polymer for the purpose of improving the various physicalproperties thereof besides the mere operations of polymerization anddrying. Particularly in the field of absorbent resins, various surfacecross-linking agents and reaction conditions for surface cross-linkinghave been proposed because cross-linking near the surface region exertsnumerous effects on the physical properties of the absorbent resin.

[0039] The methods for surface cross-linking the acrylate polymer by theuse of specific surface cross-linking agents heretofore known to the artinclude, for example, a method using a polyhydric alcohol(JP-A-58-180,233 and JP-A-61-16,903), a method using an alkylenecarbonate (DE-4020780C), a method using glyoxal (JP-A-52-117,393), amethod using a polyvalent metal (JP-A-51-136,588, JP-A-61-257,235, andJP-A-62-7,745), and a method using a silane coupling agent(JP-A-61-211,305, JP-A-61-252,212, and JP-A-61-264,006). The methods forsurface cross-linking the resin under specific reaction conditions whichhave been hereto-fore known to the art include, for example, a methodeffecting the desired cross-linking by dispersing an absorbent resin ina mixed solvent consisting of water and a hydrophilic organic solvent(JP-A-57-44,617), a method effecting the cross-linking by dispersing anabsorbent resin in an inert medium in the presence of a specific amountof water (JP-A-58-117,222), a method effecting the cross-linking byestablishing co-existence of an inorganic powder and water (U.S. Pat.No. 45687308), and a method resorting to exposure of the polymer to anelectromagnetic radiation (JP-A-63-43,930).

[0040] The various methods thus proposed, however, are invariablyincapable of sufficiently improving the various physical properties ofthe acrylate polymer by surface cross-linking. Thus, studies are stillcontinuing to attain thorough improvement of the physical properties.

[0041] In the process of studying an improvement of the surfacecross-linking of the acrylate polymer, we have found the heretoforetotally unknown fact that surface cross-linking markedly increases theresidual monomer content in the acrylate polymer to a level in theapproximate range of some tens of ppm to some hundreds of ppm and thatthe increase of the residual monomer content due to surfacecross-linking accounts for a large proportion of the residual monomercontent in the final product.

[0042] In a hydrophilic resin of an acrylate polymer which does noteasily allow a decrease of the residual monomer content and has thepeculiar phenomenon of newly generating residual monomer or increasingthe amount of already existent residual monomer in the polymer duringmanufacture of the resin or during use of the resin, this invention hasan object of providing a method for the production of a hydrophilicresin having excellent physical properties, having only a small residualmonomer content, and showing virtually no sign of generation oraugmentation of residual monomer after polymerization.

[0043] In a hydrophilic resin of an acrylate polymer which has variousphysical properties improved by cross-linking the surface regionthereof, this invention has another object of providing a method for theproduction of a hydrophilic resin which has the surface region thereofcross-linked and is consequently able to have a noticeable effect inimproving various physical properties and enjoys a notable decrease inthe residual monomer content thereof.

SUMMARY OF THE INVENTION

[0044] The objects described above are attained by a method for theproduction of a hydrophilic resin which comprises polymerizing awater-soluble unsaturated monomer containing 50 to 100 mol % of anacrylate having a neutralization ratio in the range of 30 to 100 mol %and not more than 1,000 ppm of β-hydroxy propionic acid (salt).

[0045] These objects are also accomplished by a method for theproduction of a hydrophilic resin which comprises polymerizing awater-soluble unsaturated monomer containing 50 to 100 mol % of anacrylate having a neutralization ratio in the range of 30 to 100 mol %and not more than 1,000 ppm of β-hydroxy propionic acid (salt) andtreating the surface region of the resultant acrylate polymer with asecond cross-linking agent having in the molecular unit thereof at leasta group capable of reacting with the functional group of theaforementioned acrylate polymer thereby cross-linking the surfaceregion.

[0046] These objects are also accomplished by an acrylic acid polymercomposition which contains 1-1000 ppm of β-hydroxy propionic acid (salt)and not more than 100 ppm of a residual monomer.

[0047] As a result of a diligent study made for the purpose ofaccomplishing the objects described above, we have ascertained the factthat a hydrophilic resin having a high residual monomer content and ahydrophilic resin in which the residual monomer greatly increasescontain about several 1000 ppm to 1% of β-hydroxy propionic acid (salt)as a trace element in addition to the residual monomer. We have alsofound that the β-hydroxy propionic acid content and the residual monomercontent are related.

[0048] As a means to solve the problem, we have taken note of theβ-hydroxy propionic acid (salt) contained in a small amount in theacrylic acid (or salt thereof) monomer. This correlation has never drawnany attention to date. We have succeeded in solving the various problemsmentioned above by adopting a measure to control the content of thistrace element. This invention has been perfected as a result.

[0049] The method of production contemplated by this invention has thefollowing characteristic features (1) to (5).

[0050] (1) The residual monomer content which has heretofore beendecreased by a complicated treatment such as by the use of an additivewith a resulting sacrifice in its properties, productivity, cost, andsafety of the acrylate polymer, can now be decreased by a simpleprocedure.

[0051] (2) Since the increase of the residual monomer content in thepolymer during production is small, even when an elevated temperature isused in the process a polymer can be obtained with a high operationalefficiency. Further, since the reaction can be carried out at hightemperatures, a large number of physical properties superior in highabsorption capacity of the polymer can be notably improved.

[0052] (3) Since there is little possible generation and increase of theresidual monomer content in the polymer during protracted use or at anelevated temperature, the product enjoys a high level of safety underall conditions such as, for example, those involved in protractedagronomic use and in use at the elevated temperature of hot water.

[0053] (4) The residual monomer content of the polymer can be decreasedfor the purpose of enhancing the properties of the polymer even with asmall amount of catalyst or under mild polymerization conditions, and anacrylate polymer can be obtained with still better performance withoutsacrificing physical properties.

[0054] (5) Virtually no increase of the residual monomer content in thepolymer is observed while the polymer is undergoing surfacecross-linking treatment and the effect of the surface treatment can bealso improved.

[0055] The acrylate polymer obtained as described above can be utilizedextensively in various fields covering, for example, sanitary materials,foodstuffs, civil engineering, and agriculture.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0056] Now, this invention will be described more specifically below.

[0057] For this invention, it is essential that the proportion of theacrylate in the water-soluble unsaturated monomer to be used forpolymerization be in the range of 50 to 100 mol %. The term “acrylate”as used in this invention refers to the total of acrylic acid and anacrylate.

[0058] Since the effects of this invention in lowering the residualmonomer content of the polymer is produced proportionately as theproportion of acrylate in the water-soluble unsaturated monomerincreases, the proportion of the acrylate in the water-solubleunsaturated monomer of this invention is preferably in the range of 70to 100 mol %, more preferably in the range of 90 to 100 mol %. If thecontent of the acrylate is less than 50 mol %, the shortage of supplyhas the disadvantage in not only degrading the quality of the productand boosting the cost of production but also making it difficult toproduce the effects of this invention and, depending on situation,increasing rather than decreasing the residual monomer content.

[0059] It is also essential that the neutralization ratio of theacrylate which is used for the preparation of the water-solubleunsaturated monomer be in the range of 30 to 100 mol %. If this ratio isless than 30 mol %, the produced acrylate polymer possibly producesexcessive acidity and suffers from an increase of the residual monomercontent. Conversely, if the ratio of neutralization exceeds 100 mol %,the produced acrylate polymer has the disadvantage of producingexcessive basicity and an increase of the residual monomer content.

[0060] The effect which the water-soluble unsaturated monomer used inthis invention brings about in lowering the residual monomer content ofthe produced polymer is produced more noticeably when neutralization ofthe monomer is partial than perfect neutralization. When thewater-soluble unsaturated monomer of this invention is used, the ratioof neutralization at which the residual monomer content is lowered ascompared with the conventional method is preferably in the range of 40to 95 mol %, more preferably in the range of 50 to 85 mol %. Theexpression “ratio of neutralization of the acrylate” as used in thisinvention refers to the ratio of neutralization of the acrylate used forthe preparation of the water-soluble unsaturated monomer or to the ratioof neutralization of the acrylate in the water-soluble monomer afterthis monomer has been mixed, as the situation demands, with othermonomer.

[0061] The water-soluble unsaturated monomer used for this inventioncontains 50 to 100 mol % of an acrylate having a ratio of neutralizationin the range of 30 to 100 mol %. Optionally, it may use 0 to 50 mol % ofa hydrophilic unsaturated monomer and/or a hydrophobic unsaturatedmonomer in addition to the acrylate mentioned above.

[0062] As concrete examples of the hydrophilic unsaturated monomer whichcan be used optionally in an amount in the range of 0 to 50 mol %, acidgroup-containing hydrophilic unsaturated monomers such as methacrylicacid, maleic acid, maleic anhydride, fumaric acid, crotonic acid,itaconic acid, vinyl sulfonic acid, styrene sulfonic acid,2-(meth)acrylamide-2-methylpropane sulfonic acid, 2-(meth)acryloylethane sulfonic acid, and 2-(meth)acryloyl propane sulfonic acid andsalts thereof; nonionic hydrophilic unsaturated monomers such asacrylamide, methacrylamide, N-ethyl (meth)acrylamide, N-n-propyl(meth)acrylamide, N-isopropyl .(meth)acrylamide, N,N-methyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxy poly-ethylene glycol (meth)acrylate,polyethylene glycol (meth)-acrylate, vinyl pyridine, N-vinylpyrrolidone, and N-acryloyl piperidine; and cationic hydrophilicunsaturated monomers such as N,N-dimethyl amino ethyl (meth)acrylate,N,N-diethyl amino ethyl (meth)acrylate, N,N-dimethyl amino propyl(meth)acrylate, N,N-dimethyl amino propyl (meth)acrylamide, andquaternary salts thereof may be cited. One member or a combination oftwo or more members selected from the group of hydrophilic unsaturatedmonomers cited above may be used. Such an unsaturated monomer as methyl(meth)acrylate, ethyl (meth)acrylate, or vinyl acetate which forms ahydrophilic resin by hydrolysis of the functional group thereof may beused herein.

[0063] Among other examples cited above of the hydrophilic unsaturatedmonomer which is used in an amount in the range of 0 to 50 mol % in thisinvention, methacrylic acid (salt thereof), 2-(meth)acryloyl ethanesulfonic acid (salt thereof), 2-(meth)-acrylamide-2-methyl propanesulfonic acid (salt thereof), methoxy polyethylene glycol(meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, and acrylamideprove to be particularly preferable.

[0064] As concrete examples of the hydrophobic unsaturated monomer whichis optionally used in an amount in the range of 0 to 50 mol %, styrene,vinyl chloride, butadiene, isobutene, ethylene, propylene, stearyl(meth)acrylate, and lauryl (meth)acrylate may be cited. The hydrophobicunsaturated monomer is used in an amount in the range of 0 to 50 mol %,preferably 0 to 20 mol %, and more preferably 0 to 10 mol %, based onthe total amount of monomers.

[0065] The neutralization of the water-soluble unsaturated monomercontaining 50 to 100 mol % of an acrylate having a ratio ofneutralization in the range of 30 to 100 mol % may be carried out duringthe preparation of the water-soluble unsaturated monomer or during orafter the polymerization.

[0066] In this invention, for the preparation of the water-solubleunsaturated monomer containing 50 to 100 mol % of an acrylate having aratio of neutralization in the range of 30 to 100 mol % as describedabove, it is essential that the produced water-soluble unsaturatedmonomer should contain β-hydroxy propionic acid and a salt thereof(hereinafter both collectively referred to simply as “β-hydroxypropionic acid”) in an amount of not more than 1,000 ppm, based on theamount of the water-soluble unsaturated monomer.

[0067] For this invention, the content of the β-hydroxy propionic acidin the water-soluble unsaturated monomer should be not more than 1,000ppm, preferably not more than 500 ppm, and more preferably not more than300 ppm, most preferably not more than 100 ppm, based on the amount ofsolids.

[0068] The β-hydroxy proprionic acid content of the order of 1 to 10 ppmis not particularly intolerable. The effort to lower the β-hydroxyproprionic acid content to below 1 ppm proves to be rather uneconomical.

[0069] If the content of the β-hydroxy propionic acid in thewater-soluble unsaturated monomer exceeds 1,000 ppm, the acrylatepolymer to be obtained by polymerization of the water-solubleunsaturated monomer is at a disadvantage in that it has an unduly largeresidual monomer content and, while the polymer is undergoing thesubsequent heat treatment or being used at an elevated temperature, thepolymer produces the phenomenon of newly generating residual monomer orincreasing the already existent residual monomer content and, while theacrylate polymer is undergoing a surface cross-linking treatment, thispolymer prevents the surface cross-linking treatment from having theeffect of improving various physical properties of the polymer.

[0070] The reason for the association of the content of β-hydroxypropionic acid in the polymer with an increase of the residual monomercontent or the generation and growth of residual monomer in the polymerafter drying remains yet to be determined. A forced inference is thatsince the sodium salt of β-hydroxy propionic acid has a melting point(143° C.), when the polymer obtained from the monomer containing muchmore β-hydroxy propionic acid and further since the acrylate polymergenerates or increases a large amount of residual monomer even at atemperature lower than the melting point of the salts of β-hydroxypropionic acid, the β-hydroxy propionic acid in the acrylate polymer inthe solid state is far more decomposable than the β-hydroxy propionicacid in its independent form or the β-hydroxy propionic acid possiblyinhibits polymerization or promotes depolymerization of the acrylatepolymer after the polymerization step.

[0071] For this invention, the content of the β-hydroxy propionic acidin the water-soluble unsaturated monomer when polymerization isinitiated is only required to be limited to below 1,000 ppm. Theprocedure used for the preparation of the water-soluble unsaturatedmonomer of this description is not specifically limited.

[0072] One typical example of the procedure for the preparation of thiswater-soluble unsaturated monomer is cited below.

[0073] Acrylic acid is refined by distillation and then subjected toneutralization or put to use for the preparation of a monomer as soon aspossible, specifically within a period of 24 hours, preferably 12 hours,and more preferably 6 hours. In the process of neutralization, therefined acrylic acid, at least for a while, passes the state in whichthe ratio of neutraliza-tion exceeds 100 mol %. Then, the acrylic acidis used to adjust the ratio of neutralization at a prescribed level andwhen necessary a second monomer is added to obtain the monomer as thecomponent for polymerization. As soon as possible the monomer thusobtained is subjected to polymerization, specifically within a period of24 hours, preferably 12 hours, more preferably 6 hours, and particularlypreferably 2 hours, after the preparation of the monomer. The acrylicacid fresh from the refinement by distillation is preferably kept at thelowest possible temperature, specifically below 30° C., preferablybetween the solidifying point and 25° C., until it is subjected toneutralization or put to use for the preparation of the monomer. Theneutralization is preferably carried out at a low tempera-ture for abrief period. When the acrylic acid resulting from the distillation isleft standing for a relatively long time, it is preferably kept in ananhydrous state. Further, the monomer which has been prepared ought tobe stored at a tempera-ture in the range between the solidifying pointthereof and 40° C., preferably between 0° C. and 30° C. If the storagetemperatures of acrylic acid and the monomers are high, an amount ofβ-hydroxy propionic acid and the residual monomer sometimes increase, soit is not preferable.

[0074] Acrylic acid is finally distilled and put to storage at anacrylic acid manufactory and subsequently shipped out. It takes at leastfour to five days and more generally some tens of days to some monthsbefore it is put to actual use on a commercial scale by the consumer.The monomer is prepared in a large amount at a factory and then put tostorage in the plant. An average of not less than three days elapsesafter the preparation of the monomer is completed and before it is putto actual use. More-over, in preparation for the polymerization, themonomer required several hours for the deaeration treatment of andtemperature adjustment. We have found that the amounts of β-hydroxypropionic acid and residual monomer increase proportionately as the timeafter the refinement of acrylic acid by distillation increases and thetime after completion of the preparation of the monomer and before themonomer is put to polymerization increases. For the present invention,therefore, the monomer fresh monomer is preferably produced just afterdistillation in a short time, and also the monomer fresh from itspreparation is preferably polymerized as soon as possible.

[0075] Of the various methods which are available for neutralizingacrylic acid under the condition of causing the acrylic acid at leastfor a while to pass the state in which the ratio of neutralizationexceeds 100 mol %, (1) the most convenient method comprises keeping thesystem of neutralization reaction cooled and simultaneously graduallyadding a acrylic acid to a fixed amount of basic substance.

[0076] As another method which is available for the same purpose, (2)the method as disclosed in JP-A-2-209,906 and EP-A-0372706 may be cited.The method comprises allowing the ratio of neutralization of theacrylate in the system of neutralization to remain below 100 mol % fromthe start of the neutralization, then causing the acrylate in theprocess of neutralization to pass the stage in which the ratio ofneutrali-zation exceeds 100 mol %, and finally adjusting the ratio ofneutralization to a level in the range of 30 to 100 mol %. The method of(1) sacrifices the velocity of neutraliza-tion and necessitates a lowtemperature for the decrease of β-hydroxy propionic acid. The method of(2) fails to attain a decrease of β-hydroxy propionic acid content withhigh operational efficiency so long as it uses the aforementionedacrylic acid.

[0077] From the standpoint of the various physical properties of theproduced absorbent resin and the residual monomer remaining in theresin. This invention contemplates partially or wholly neutralizing suchan acid radical-containing monomer as acrylic acid.

[0078] The basic substances which can be effectively used for theneutralization of the monomer include, for example, (hydrogen)carbonates and hydroxides of alkali metals, ammonia, and various aminoacids such as alanine, and organic amines. Among other basic substances,sodium hydroxide and/or potassium hydroxide prove to be preferable andsodium hydroxide prove to be more preferable from the viewpoint of thevarious physical properties and a decrease of the residual monomercontent. Further, if ammonia is used together with NaOH/KOH in thepresent invention, the residual monomer can be more highly decreased.The monomer may incorporate therein, urea as an ammonia precursor beforeit is polymerized. The gel polymer obtained from the partiallyneutralized monomer is not necessarily prohibited from being furtherneutralized during or after polymerization. When such a strong base assodium hydroxide is used for the after neutralization, due attentionmust be paid to preclusion of the possible hydrolysis of cross-linkingpoints. When the further neutralization is to be effected by the use ofa polyester type cross-linking agent, such a weak base as ammonia or ahydrogen carbonate of an alkali metal is favorably used.

[0079] In this invention, in order to more highly decrease the residualmonomer, the proportion of an ammonium salt in the base to beneutralized mainly as an alkali metal salt and ammonium salt is in therange of 4 to 50 mol %, preferably 10 to 40 mol %. The proportion of thealkali metal salt in the base is in the range of 10 to 96 mol %,preferably 20 to 80 mol %. So long as these salts are within therelevant ranges mentioned above, the absorbent resin to be obtainedentails neither coloration nor the production of any harmful byproducts,suffers from only a small residual monomer content, and excels invarious physical properties. The neutralization with an extremely smallamount of a polyvalent metal salt constitutes in itself no alteration ofthe gist of this invention. Further, neutralization is carried by usingtogether with ammonia, in order to decrease the residual monomer, theranges specified above for the proportions of the relevant salts becomehighly significant when the gel polymer or a salt thereof is subjectedto below mentioned hear-treatment. Thus, the salts may be in amountsbelow the lower limits of the ranges in the course of polymerization andthen adjusted to the proportions falling within the limits prior to heattreatment. The adjustment of these proportions to the ranges mentionedabove is conveniently effected preparatorily to polymerization. Thisparticular timing of the adjustment is favorable in that it allows theeffect of the surface treatment to be additionally enhanced.

[0080] This invention can obtain the absorbent resin by polymerizing andcross-linking the monomer; the β-hydroxy propionic acid content of whichis not more than 1,000 ppm as mentioned above.

[0081] The method to be adopted for the cross-linking treatment is notparticularly discriminated by this invention. For example, a methodwhich comprises polymerizing the monomer of this invention therebyobtaining water-soluble resin and then cross-linking this resin in thepresence of a cross-linking agent added to the resin during or after thepolymerization, a method which effects radical cross-linking by the useof a radical polymerization initiator, and a method which effectsradical cross-linking by the use of an electron beam may be cited. Forthe purpose of obtaining an absorbent resin of excellent quality withhigh operational efficiency, the procedure which comprises polymerizingthe monomer to which a cross-linking agent has been added in aprescribed amount prior to polymerization and subjecting the resultantpolymer to a cross-linking reaction simultaneously with or subsequentlyto polymerization proves to be particularly preferred.

[0082] As typical examples of the cross-linking agent used in the methodwhich comprises polymerizing the monomer in the presence of across-linking agent added in a prescribed amount to the monomer inadvance of polymerization and subjecting the resultant polymer to across-linking treatment either simultaneously with or subsequently tothe polymerization, N,N′-methylene bis-acrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propy-lene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylol propane di(meth)acrylate,(poly)-ethylene glycol di(β-acryloyloxy propionate), trimethylol propanetri(β-acryloyloxy propionate), poly(meth)allyloxy alkane, (poly)ethyleneglycol diglycidyl ether, ethylene glycol, polyethylene glycol, glycerin,pentaerythritol, ethylene diamine, and polyethylene imine may be cited.The amount of the cross-linking agent to be used is generally in therange of 0.005 to 5 mol %, preferably 0.01 to 1 mol %, based on theamount of monomer. Among the cross-linking agents cited above, it ispreferable to use essentially a polymerizing cross-linking agent whichhas two or more polymerizing unsaturated groups in the molecular unitthereof subject to the durability and absorption characteristics of theabsorbent resin to be obtained and the convenience of handling of thehydrated gel during the course of production.

[0083] When the monomer is to be polymerized as described above in thisinvention, although either bulk polymerization or precipita-tionpolymerization may be employed, from the viewpoint of the quality of theproduct and the ease of control of the polymerization step the monomeris preferably polymerized in the form of a solution. The solvent for thepolymerization system has no particular restriction except for the solerequirement that it should be a liquid capable of dissolving themonomer. As concrete examples of the solvent, water, methanol, ethanol,acetone, dimethyl formamide, and dimethyl sulfoxide may be cited. Amongother solvents which are available at all, water or an aqueous liquidproves to be particularly preferred. Although the concentration of themonomer in the solution may exceed that of a saturated solution, it isgenerally in the range of 20% by weight to of the concentration of asaturated solution, preferably in the range of 25 to 50% by weight. Ifthe concentration of the monomer is unduly high, due attention should bepaid to the avoiding of the otherwise possible degradation of variousphysical properties of the product.

[0084] The monomer ready for polymerization may incorporate therein suchwater-soluble chain. transfer agents as hypophosphites and saltsthereof, thiols, and thiolic acid and salts thereof and such hydrophilicpolymeric compounds as starch, cellulose, polyvinyl alcohol, polyacrylicacid, and cross-linked polyacrylic acid. Generally, the amount of awater-soluble chain transfer agent is within 5 parts by weight and thatof a hydrophilic polymeric compound within 50 parts by weight.

[0085] The water-soluble unsaturated monomer having a β-hydroxypropionic acid content of not more than 1,000 ppm which has beenobtained as described above is then subjected to polymerization.Further, the water-soluble unsaturated monomer after finishing controlis preferably subjected to polymerization as short time as possible asmentioned above.

[0086] As typical examples of the method to be adopted by this inventionfor polymerization, radical polymerization using a radicalpolymerization initiator and polymerization using an active energy raysuch as an ultraviolet light or an electron beam may be cited. In allthese methods, radical polymerization by virtue of a radicalpolymerization initiator is preferred for the purpose of obtaining anacrylate polymer which excels in quality.

[0087] By the use of the method of production according to thisinvention, the water-soluble unsaturated monomer is allowed to acquireimproved polymerizability and a decrease of the residual monomer contentcan be obtained with a radical polymerization initiator used in asmaller amounts than normally required. Thus, the polymerization can becontrolled easily and the acrylate polymer can be produced with stillbetter properties.

[0088] The radical polymerization initiator used may be any of thoseknown in the art. As typical examples of the radical polymerizationinitiator which can be used effectively, persulfates such as potassiumpersulfate, ammonium persulfate, and sodium persulfate; organicperoxides such as t-butyl hydroperoxide and cumene hydroperoxide;hydrogen peroxide; azo compounds such as 2,2′-azo-bis(2-amidinopropane)dihydrochloride; and chlorites, hypochlorites, ceric salts, andpermanganates may be cited. Among other radical polymerizationinitiators cited above, it is preferable to use one or more membersselected from the group consisting of persulfates, hydrogen peroxide,and azo compounds from the viewpoint of the quality of the producedacrylate polymer and decreasing of the residual monomer content.

[0089] When an oxidizing radical polymerization initiator is used as thepolymerization initiator in this invention, it may be used incombination with a reducing agent to effect the reaction in the form ofredox polymerization. As concrete examples of the reducing agent usedfor this purpose, (hydrogen) sulfites such as sodium sulfite and sodiumhydrogen sulfite; thiosulfates such as sodium thiosulfate; dithionites;metal salts such as cuprous sulfate and ferrous sulfate; organicreducing agents such as L-ascorbic acid; and amines such as aniline andmonoethanol amine may be cited.

[0090] The radical polymerization initiator may be added all at once orgradually added to the polymerization system. The amount of the radicalpolymerization initiator to be used is generally in the range of 0.001to 2 mol %, preferably 0.01 to 1 mol %, based on the amount of thewater-soluble unsaturated monomer.

[0091] The radical polymerization for the production of the acrylatepolymer may be performed by any of the known techniques. As typicalexamples of the method of radical polymerization, various forms ofaqueous solution polymerization such as cast polymerization performed ina molding frame (JP-B-48-42,466), polymerization to be performed on abelt conveyor (JP-A-58-49, 714), polymerization performed on a finelydivided hydrogel polymer (JP-A-57-34,101, U.S. Pat. No. 4,625,001 andU.S. Pat. No. 5,124,416), and polymerization performed under pressure(JP-A-2-129,207), reversed-phase suspension polymerization(JP-B-59-37,003), reversed-phase emulsion polymerization (JP-A-63-90,510and JP-A-63-90,537), polymerization of a monomer complexed with afibrous substrate (JP-A-2-242,975), precipitation polymerization(JP-A-58-84,819, JP-A-1-1,710, and JP-A-1-204,910), and bulkpolymerization may be cited.

[0092] When the water-soluble unsaturated monomer is polymerized in theform of a solution, the acrylate polymer resulting from thepolymerization may be put to use in its unmodified form. However, it is,preferably used in a dried form for the purpose of enhancing itsconvenience of handling and decreasing the residual monomer content.

[0093] The drying of the acrylate polymer in this invention may becarried out by any of the methods of drying known to the art. Forexample, a method which comprises polymerizing a monomer at a highconcentration thereby simultaneously effecting the drying andpolymerization by virtue of the heat of polymerization (JP-A-58-71,907and JP-A-2-34,607) may be adopted. The produced gel polymer may befurther dried, depending on the solids content of the produced polymer.

[0094] The methods which maybe effectively used for the further dryingof the gel polymer include, for example, drying under conditions of highhumidity (JP-A-1-26,604), azeotropic dehydration in an organic solvent,drying by means of microwave, drying using a belt or a drum drier heatedto a prescribed temperature, drying in a cylinder furnished with ahigh-speed rotor (JP-A-2-240,112), and drying by the use of a forceddraft oven, an infrared ray, or a vacuum drying device.

[0095] The temperature for drying the gel polymer in this invention isgenerally in the range of 70° to 300° C., preferably 110° to 260° C.,and more preferably 150° to -250° C. If this temperature is less than70° C., the drying takes an unduly long time. If the temperature exceedsthis range, decreasing effect of the residual monomer by using themomers in accordance of the present invention is defficult to occur.Particularly when the acrylate polymer is an absorbent resin, the dryingperformed at a temperature in the range of 110° to 260° C., preferably150° to 250° C. is preferred in that not only the reduction of residualmonomer but also the absorption ratio is enhanced. The time for thedrying operation is suitably decided by the water content and theparticle diameter of the gel polymer and the temperature of drying. Itis generally in the range of one minute to 10 hours, preferably 10minutes to five hours.

[0096] The drying performed at an elevated temperature has beenheretofore found to be preferable from the standpoint of operationalefficiency. However, it necessarily involves an increase in the residualmonomer content. In the process of searching for the cause of thisinevitable increase in the residual monomer content, we have found thisincrease in the residual monomer content occurs while the drying is inprocess. Thus, the problem of the increase in the residual monomercontent has been solved. To be specific, the use of the water-solubleunsaturated monomer according to this invention enables an acrylatepolymer having only a small residual monomer content and excellentphysical properties such as excellent absorption capacity to be obtainedwithout reference to the temperature of drying and the time for drying.

[0097] Further, ammonia is used for neutralization, it is preferred thatthe polymer is subjected to heat treatment in order to decrease theresidual monomer content.

[0098] The state to be assumed by the polymer prior to heat treatment isnot particularly discriminated by this invention. For example, thepolymer may be in the form of a gel fresh from polymerization, adispersion in an organic solvent, or a dry solid. The solids content ofthe polymer prior to heat treatment may be kept at a constant level ormay be increased by vaporizing the solvent. For example, specifically,in regard to the timing for carrying out the heat treatment, the step ofdrying, the step of surface cross-linking, the step of reheating afterdrying, the step of pelletizing, and the step of using additives may becited. The heat treatment which is performed at the step of drying atthe above mentioned temperature proves to be particularly preferred.Specifically if the gel polymer of this invention is an absorbent resin,the enhancement of the absorption ratio and the notable decrease of theresidual monomer content are accomplished by drying the gel polymer atthe temperature mentioned above and subjecting it to heat treatment. Themethod adopted for the drying is not particularly discriminated by thisinvention. Such known methods of drying as hot air drying, infrared raydrying, and azeotropic dehydration are concrete examples. Further, whenammonia is used together, ammonia used for neutralization apt tovolatile at heat treatment, it is necessary to pay attention to odorduring a process and control of properties of the polymer along withdecrease of neutralization ratio.

[0099] The absorbent resin freshly polymerized or dried may incorporatetherein such additives as a surfactant, an inorganic fine powder, and a(hydrogen) sulfite. It may be otherwise pulverized or pelletized for thepurpose of adjustment of particle size. When a powder absorbent resin isto be obtained, the absorbent resin particle size is adjusted to anaverage particle diameter in the range of 10 to 1,000 μm, morepreferably 100 to 1,000 μm, and most preferably 300 to 600 μm.

[0100] The acrylate polymer having a cross-linked surface region whichis provided by this invention can be obtained by cross-linking thesurface region of the acrylate polymer which has been produced by themethod described above.

[0101] When the acrylate polymer which is obtained from a water-solubleunsaturated monomer having a β-hydroxy propionic acid content of notless than 1,000 ppm is used as the acrylate polymer intended to befurnished with a cross-linked surface region, the surface cross-linkinginduces an increase in the residual monomer content and exerts anadverse effect on the safety of the product.

[0102] As respects the water content of the acrylate polymer used inthis invention, though the acrylate polymer resulting from thepolymerization may be put to use in its undried form, it is generallypreferred to use the acrylate polymer in its dried form from thestandpoint of the effect of the surface cross-linking. To be specific,the acrylate polymer which has a cross-linked surface region ispreferably dried until the water content thereof falls below 40%,preferably below 30%, and more desirably below 10%.

[0103] Regarding the particle size of the acrylate polymer to be used inthis invention, preferably the average particle diameter is in theapproximate range of 10 to 2,000 μm, more preferably 100 to 1,000 μm,and most preferably 300 to 600 μm. The particle size distribution ofthis acrylate polymer is preferred to be as narrow as possible.

[0104] The acrylate polymer used herein may be a water-soluble resin.The effect of the surface cross-linking of this invention brought aboutin improving various physical properties of the polymer is produced morenoticeably in the acrylate polymer in the form of an absorbent resin.

[0105] The second cross-linking agent used for cross-linking the surfaceregion of the acrylate polymer in this invention may be any of the knowncross-linking agents which generally find extensive utility in numerousapplications. As concrete examples of the second cross-linking agent,polyhydric alcohols, polyepoxy compounds, polyamines, polyaziridines,polyaldehydes, polyisocyanates, polyoxazolines, alkylene carbonates, andpolyvalent metals may be cited. The compounds which have such functionalgroups are other examples.

[0106] The polyhydric alcohols which may be effectively used include,for example, ethylene glycol, diethylene glycol, propylene alcohol,triethylene glycol, tetraethylene glycol, polyethylene glycol, propyleneglycol, 1,3-propane diol, dipropylene glycol,2,2,4-trimethyl-1,3-pentadiol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,4-butane diol, 1,5-pentane diol,1,6-hexane diol, 1,2-cyclohexane dimethanol, 1,2-cyclohexanol,trimethylol propane, diethanol amine, triethanol amine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol,and sorbitol.

[0107] The polyepoxy compounds which may be effectively used include,for example, ethylene glycol diglycidyl ether, diglycerol polyglycidylether, polyglycerol polyglycidyl ether, propylene glycol diglycidylether, and polypropylene glycol diglycidyl ether.

[0108] The polyamines which may be effectively used include, forexample, ethylene diamine, diethylene triamine, triethylene tetramine,tetraethylene pentamine, pentaethylene hexamine, and polyethylene imine.

[0109] The polyaziridines which may be effectively used include, forexample, 2,2-bis-hydroxymethyl butanol tris(3-(1-aziridinyl)propionate), 1,6-hexanemethylene diethylene urea, anddiphenyl-methane-bis-4,4-N,N′-diethylene urea.

[0110] The polyaldehydes which may be effectively used include, forexample, glyoxal and glutar aldehyde.

[0111] The polyisocyanates which may be effectively used include, forexample, 2,4-tolylene diisocyanate and hexamethylene diisocyanate.

[0112] The polyoxazolines which may be effectively used include, forexample, 1,2-ethylene bis oxazoline and polyisopropenyl oxazoline, forexample.

[0113] The alkylene carbonates which may be effectively used include,for example, 1,3-dioxolan-2-on, 4-methyl-1,3-dioxolan-2-on,4,5-dimethyl-1,3-dioxolan-2-on, 4,4-dimethyl-1,3-dioxolan-2-on,4-ethyl-1,3-dioxolan-2-on, 4-hydroxymethyl-1,3-dioxolan-2-on,1,3-dioxan-2-on, 4-methyl-1,3-dioxan-2-on,4,6-dimethyl-1,3-dioxane-2-on, and 1,3-dioxoban-2-on.

[0114] The haloepoxy compounds which may be effectively used include,for example, epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin.

[0115] The polyvalent metals which may be effectively used as a surfacecross-linking agent herein include, for example, hydroxides andchlorides of zinc, calcium, magnesium, aluminum, iron, and zirconium.

[0116] As the cross-linking agent for use in this invention, it ispreferable to adopt one member or a combination of two or more membersselected from the group of cross-linking agents mentioned above. In thelight of the effect of surface cross-linking, it is preferable to use asthe cross-linking agent one member or a combination of two or moremembers selected from the group consisting of polyhydric alcohols,polyglycidyl compounds, poly-amines, and alkylene carbonates.Particularly from the viewpoint of not only the effect of surfacecross-linking but also the safety and cost, it is most preferable to usea polyhydric alcohol as the surface cross-linking agent.

[0117] The amount of the surface cross-linking agent to be used in thisinvention, although variable with the type of cross-linking agent used,is generally in the range of 0.001 to 20 parts by weight, preferably0.01 to 10 parts by weight, based on 100 parts by weight of the solidsof the acrylate polymer obtained by this invention. So long as thisamount is in the range just mentioned, the acrylate polymer having across-linked surface region can be obtained with excellent physicalproperties. If the amount of the surface cross-linking agent to be usedexceeds 20 parts by weight, the excess not merely impairs the economicsof the production but also constitutes in itself an extravagance foraccomplishing the proper effect of cross-linking. If this amount is sosmall as to fall short of 0.001 part by weight, the surfacecross-linking does not easily bring about an improvement of the variousphysical properties.

[0118] In this invention, water may be used in mixing the acrylatepolymer with the cross-linking agent. In this invention, the amount ofwater to be used is not more than 20 parts by weight, preferably in therange of 0.5 to 10 parts by weight, based on 100 parts by weight of thesolids of the acrylate polymer, depending on the type, particle size,and water content of the acrylate polymer.

[0119] In this invention, a hydrophilic organic solvent may be used inmixing the cross-linking agent and the acrylate polymer. The hydrophilicorganic solvents which may be effectively used include, for example,lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol,isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butylalcohol; ketones such as acetone; ethers such as dioxane andtetrahydrofuran; amides such as N,N-dimethyl formamide; and sulfoxidessuch as dimethyl sulfoxide. Though the optimum amount of the hydrophilicorganic solvent used in this invention varies with the type and particlesize of the acrylate polymer, it is generally in the range of 0 to 10parts by weight, preferably 0 to 8 parts by weight, based on 100 partsby weight of the solids of the acrylate polymer.

[0120] In this invention, the acrylate polymer and the cross-linkingagent (and water/hydrophilic organic solvent) may be mixed either in anon-dispersed system or in a dispersed system.

[0121] The mixture of the acrylate polymer with the cross-linking agentin the non-dispersed system is effected, for example, by such a methodas disclosed in DE-A-4020780. This method comprises directly sprayingthe cross-linking agent or the mixed liquid of the cross-linking agentand water and/or a hydrophilic organic solvent onto the acrylate polymeror adding it dropwise thereto. When water is used in the mixtureeffected in the non-dispersed system, the mixing may be performed in thepresence of an inorganic compound or incorporating a surfactant for thepurpose of ensuring uniform dispersion of the cross-linking agent in theproduced mixture (U.S. Pat. No. 4,587,308 and EP-A-0509706). Optionally,the incorporation of water in the process of mixing may be carried outin the form of steam (JP-A-1-297,430).

[0122] As concrete examples of the method for mixing the acrylatepolymer with the cross-linking agent in the dispersed system, a methodwhich comprises dispersing the acrylate polymer in a hydrophilic organicsolvent (JP-A-57-44,627) and a method which comprises dispersing theacrylate polymer in a hydrophobic solvent (JP-A-59-62,665) may be cited.The surface cross-linking treatment in the dispersed system is alsopreferably carried out in the presence of a prescribed amount of water(JP-A-58-117,222).

[0123] After the acrylate polymer has been mixed with the cross-linkingagent in the non-dispersed system or dispersed system, the resultantmixture is further heated to have the surface region thereofcross-linked, depending on the type of cross-linking agent. Generally,it is desirable to heat-treat this mixture for the purpose of promotingthe cross-linking reaction and, at the same time, volatilizing theorganic solvent and water which have been added optionally in theprocess of the mixing.

[0124] As concrete examples of the method for performing this heattreatment, a method which comprises directly heating the acrylatepolymer which has a surface cross-linking agent incorporated therein inthe non-dispersed system, a method which comprises subjecting theacrylate polymer having a cross-linking agent incorporated therein to aheat treatment per se in the dispersed system, and a method whichcomprises removing the acrylate polymer by filtration from the dispersedsystem and heat-treating the separated acrylate polymer in thenon-dispersed system may be cited.

[0125] In this invention, when the heat treatment is carried out afterthe addition of the cross-linking agent, the temperature of this heattreatment is generally not less than 75° C., preferably in the range of100° C. to 300° C., more preferably 120° to 260° C., and most preferably150° to 250° C. If the temperature of this heat treatment is unduly low,decreasing effect of the residual monomer by using the monomers inaccordance with the present invention is difficult to occur, and theheat treatment takes an excessively long time and the operationalefficiency of heat treatment is degraded and uniform and fast surfacecross-linking cannot be easily attained. Conversely, if the temperatureis so high as to exceed 300° C., the acrylate polymer maybe subject tothermal deterioration. The duration of the heat treatment is suitablydecided with due consideration to the desired effect of the surfacetreatment and the temperature of heat treatment.

[0126] Heretofore, the practice of performing the heat treatment at ahigh temperature, for example, falling in the range of 120° to 260° C.has been advocated for the purpose of allowing the surface cross-linkingto proceed uniformly and quickly. We have found that the heat treatmentperformed at this elevated temperature increases the residual monomercontent in the acrylate polymer to a level in the range of some tens tosome thousands of ppm. The acrylate polymer according to the presentinvention, however, shows virtually no increase in the residual monomercontent during the heat treatment of the surface cross-link irrespectiveof the temperature of heating or the duration of heating. And when thepolymer is neutralized using ammonia, the residual monomer contentrather decreases. The conventional heat treatment has failed to effectsufficient improvement in the various physical properties of the polymerperhaps because of the generation and growth of residual monomer in thepolymer during the surface treatment.

[0127] So long as the acrylate polymer described above is adopted, thetreatment with a surface cross-linking agent and the heat treatmentenable the acrylate polymer to acquire an improved quality and form across-linked texture in the surface region thereof and allows virtuallyno increase in the residual monomer content irrespective of the methodadopted for the addition of the surface cross-linking agent or themethod of heat treatment. Since the addition of the cross-linking agentin the dispersed system and the heat treatment in the dispersed systemrequire a large amount of organic solvent, the heat treatment ispreferably carried out in the non-dispersed system from for the benefitof the operational efficiency and the safety of the heat treatment.Further from the standpoint of the quality of the produced resin, notonly the heat treatment but also the addition of the surfacecross-linking agent is preferably carried out in the non-dispersedsystem.

[0128] Further the absorbent resin in accordance with the presentinvention has not more than 100 ppm, preferably not more than 10 ppm ofresidual monomer content in substantial dry state and is an absorbentresin which contains acrylic acid (salt) containing not more than 100ppm, preferably not more than 10 ppm of residual monomer content afterheating at a temperature of 180° C. for 3 hours.

[0129] In a conventional absorbent resin, the residual monomer contentis merely decreased apparently, but a large amount of residual monomergenerates by heating. However, the absorbent resin in accordance withthe present invention has a residual monomer content of not more than100 ppm, ever after heating at a temperature of 180° C. for 3 hours andthe residual monomer content sometime rather decreases and safe underany condition. This is based on the reason that the acrylic acid saltpolymer in accordance with the present invention is a novel acrylic acidsalt polymer composition in which -hydroxy propionic acid content isextremely decreased, for example, 1 to 1000 ppm, preferably 1-500 ppm,more preferably 1 to 100 ppm compared to a conventional polymer.

[0130] Now, this invention will be described more specifically belowwith reference to the working examples. It should be noted, however,that the scope of this invention is not limited to these workingexamples in any respect. The physical properties of the acrylate polymerwhich will be described in these working examples represents themagnitudes determined by the following testing methods.

[0131] The term “latent residual monomer content” as used herein refersto the residual monomer which occurs as an increment produced byprotracted use of the polymer or exposure of the polymer to an elevatedtemperature to the residual monomer generally found in the acrylatepolymer at normal room temperature. Of course, a the latent residualmonomer content is large notwithstanding the fact that the apparentresidual monomer content is small is undesirable from the standpoint ofsafety.

[0132] (1) Absorption Capacity

[0133] This property is determined by placing 0.2 g of a given acrylatepolymer uniformly in a teabag-like pouch (40×150 mm) made of non-wovenfabric, keeping the pouch containing the sample immersed in an aqueous0.9 wt % sodium chloride solution for 30 minutes, removing the pouchfrom the solution, allowing the drenched pouch to drain for a prescribedtime, weighing the wet pouch, and calculating the following formulausing the weight.

Absorption ratio (g/g)=((Weight of wet pouch)−(Weight of wet blankpouch))/(Weight of absorbent resin)

[0134] (2) Residual Monomer Content

[0135] This magnitude is determined by stirring a dispersion of 0.5 g ofa given acrylate monomer in 1,000 ml of deionized water for two hours,passing the resultant dispersion through a Wattman filter paper, andassaying the filtrate for residual monomer content by means ofhigh-speed liquid chromatography.

[0136] (3) Latent Residual Monomer Content

[0137] A given acrylate polymer is heated at 180° C. for three hours- tofacilitate analysis thereof for the latent residual acrylic acidcontent. The acrylate polymer which has undergone this heat treatment istested for the residual monomer content by the method of (2) above. Theincreasement produced in the residual monomer content as a consequenceof the heat treatment is reported as the latent residual monomercontent.

[0138] (4) Suction Power

[0139] This property is determined by preparing a petri dish having theinner bottom thereof covered with tissue paper, pouring 20 ml ofartificial urine (containing 1.9% of urea, 0.8% of NaCl, 0.1% of CaCl₂,and 0.1% of MgSO₄) into the petri dish, dropping 1 g of a given acrylatepolymer having a cross-linked surface region at the center of the petridish, allowing the sample to absorb the artificial urine through thetissue paper for 10 minutes, weighing the swollen gel, and determiningthe increase in the weight of the sample. This increase is reported asthe magnitude of the suction power of the sample.

Acrylate to be Used

[0140] The acrylates used in the following working examples and controlswere products obtained by the following procedure. The contents ofβ-hydroxy propionic acid in the produced acrylates and the polymers weremagnitudes expressed in ppm, based on the solids determined by liquidchromatography.

[0141] The species of acrylic acid refined by distillation were storedin a dark room at a fixed temperature in the range of 20° to 40° C.(assuming a normal room temperature) until they were in theneutralization step. As things stand, some tens of days elapse beforethe acrylic acid refined by distillation at production facilitiesreaches the end consumer engaging in the production of the hydrophilicresin. More often than not, the acrylic acid is handled in an about 80%aqueous solution having a high point of solidification so as to preventit from freezing.

Production 1

[0142] Acrylic acid obtained from an acrylic acid production plant wasrefined by distillation. The refined acrylic acid was stored at 30° C.for 3 hours and then neutralized by the following procedure of Example 1cited in EP-A-0372706.

[0143] A distillation flask provided with a stirrer was charged with2,744 g of deionized water. With the temperature of the neutralizationreaction system inside the flask kept at a level in the range of 20° to40° C., 1,390 g of the acrylic acid and 1,480 g of an aqueous 48 wt %sodium hydroxide solution were simultaneously added dropwise over aperiod of 100 minutes into the flask at a sodium hydroxide/acrylic aciddropping ratio in the range of 0.9 to 0.95. After the end of thedropwise addition, 160 g of an aqueous 48 wt % sodium hydroxide solutionwas supplied to the flask to adjust the ratio of neutralization of theneutralization reaction system inside the flask to 102 mol %. With thetemperature of the neutralizing reaction system adjusted to 40° C., theproduct of neutralization was left aging for 30 minutes. Subsequent tothe completion of the aging, 499 g of acrylic acid was supplied over aperiod of 10 minutes to the neutralization reaction system to obtain anacrylate (I) of a concentration of 37% with a ratio of neutralization of75 mol %.

[0144] The content of β-hydroxy propionic acid in the acrylate (I) wasfound to be 40 ppm based on the solids of the acrylate.

Productions 2 and 3

[0145] Acrylates (II) and (III) were obtained by following the procedureof Production 1, except that samples of acrylic acid which had beenstored at a temperature of 30° C. for 10 hours and 24 hours respectivelyafter the refinement by distillation were used for the neutralization.

[0146] The contents of β-hydroxy propionic acid in the acrylates (II)and (III) based on the solids thereof were found to be 90 ppm and 190ppm respectively.

Production 4

[0147] An acrylate (IV) was obtained by following the procedure ofProduction 1, except that a sample of acrylic acid which had been storedat a temperature of 25° C., for 24 hours after being refined bydistillation was used for the neutralization. The content of β-hydroxypropionic acid in the acrylate (IV) based on the solids thereof wasfound to be 100 ppm.

Productions 5 to 7

[0148] Acrylates (V) to (VII) were obtained by following the procedureof Production 1, except that samples of acrylic acid which had beenstored at a temperature of 20° C. for 10 hours, 24 hours, and 48 hoursrespectively after being refined by distillation were used for theneutralization. The contents of β-hydroxy propionic acid in theacrylates (V) to (VII) based on the solids thereof were found to be 50ppm, 80 ppm, and 130 ppm respectively.

Production 8

[0149] A commercially available acrylic acid (guaranteed reagentproduced by Wako Junyaku K. K. ) was refined by distillation. Therefined acrylic acid was stored at a temperature of 30° C. for 3 hoursand then neutralized by the following procedure of Control 2 cited inEP-A-0372706.

[0150] A distillation flask provided with a stirrer was charged with2,744 g of deionized water and 1,640 g of an aqueous 48 wt % sodiumhydroxide solution. Then, with the temperature of the neutralizationreaction system kept at a level in the range of 20° to 40° C., 1,889 gof the acrylic acid was supplied over a period of 120 minutes to theflask to obtain an acrylate (VIII) of a concentration of 37% at a ratioof neutralization of 75 mol %. The content of β-hydroxy propionic acidin the acrylate (VIII) based on the solids thereof was found to be 230ppm.

Productions 9 and 10

[0151] Acrylates (IX) and (X) were obtained by following the procedureof Production 8, except that samples of acrylic acid which had beenstored at a temperature of 30° C. for 10 hours and 24 hours respectivelyafter the refinement by distillation were used for the neutralization.The contents of β-hydroxy propionic acid in the acrylates (IX) and (X)based on the solids thereof were found respectively to be 290 ppm and390 ppm.

Production 11

[0152] A commercially available acrylic acid (guaranteed reagentproduced by Wako Junyaku K. K. ) was refined by distillation. Therefined acrylic acid was converted into an aqueous 80% solution having ahigh point of solidification there by allowing easy handling. Theaqueous solution was stored at a temperature of 30° C. for three hoursand then neutralized by the following procedure resembling that ofProduction 8.

[0153] A distillation flask provided with a stirrer was charged with2,272 g of deionized water and 1,640 g of an aqueous 48 wt % sodiumhydroxide solution. Then, with the temperature of the neutralizationreaction system kept at a level in the range of 20° to 40° C., 2,361 gof an aqueous 80% acrylic acid solution was supplied over a period of120 minutes to the flask to obtain an acrylate of a concentration of 37%with a ratio of neutralization of 75 mol %.

[0154] The content of β-hydroxy propionic acid in the acrylate (XI)based on the solids thereof was found to be 290 ppm.

Production 12

[0155] An acrylate (XII) was obtained by effecting neutralization inaccordance with the procedure of Production 8, except that thetemperature of neutralization was lowered to 10° C. However, due to thefall of the temperature of neutralization, the time required for theneutralization increased to six hours, a period three times the periodrequired in Example 8 of Production. The content of β-hydroxy propionicacid in the acrylate (XII) based on the solids thereof was found to be70 ppm.

Production 13

[0156] An acrylate (XIII) of a concentration of 45% having a ratio ofneutralization of 50 mol % was obtained by following the procedure ofProduction 1, except that the amount of the acrylic acid added to theneutralization reaction system subsequent to the completion of the agingat a ratio of neutralization of 102 mol % was changed from 499 g to1,446 g. The content of β-hydroxy propionic acid in the acrylate (XIII)based on the solids thereof was found to be 30 ppm.

Production 14

[0157] An acrylate (XIV) of a concentration of 34% having a ratio ofneutralization of 90 mol % was obtained by following the procedure ofProduction 1, except that the amount of acrylic acid added to theneutralization reaction system subsequent to the completion of the agingat a ratio of neutralization of 102 mol % was changed from 499 g to 184g. The content of β-hydroxy propionic acid in the acrylate (XIII) basedon the solids thereof was found to be 50 ppm.

Production 15

[0158] An acrylate (XV) of a concentration of 37% having a ratio ofneutralization of 100 mol % was obtained by following the procedure ofProduction 8, except that the amount of the deionized water was changedfrom 2,744 g of 1,942 g and the amount of acrylic acid added dropwise tothe neutralization reaction system was changed from 1,889 g to 1,418 g.The content of β-hydroxy propionic acid in the acrylate (XV) based onthe solids thereof was found to be 290 ppm.

Control Production 1

[0159] An acrylate (I) for comparison was obtained by following theprocedure of Production 1, except that acrylic acid stored at 25° C. for20 days after being refined by distillation was used instead for theneutralization. The content of β-hydroxy propionic acid in the acrylate(I) for comparison based on the solids thereof was found to be 2,500ppm.

Control Production 2

[0160] An acrylate (II) for comparison was obtained by following theprocedure of Production 1, except that a commercially available acrylicacid (guaranteed reagent produced by Kanto Kagaku K. K. ) was used forneutralization without being refined in advance by distillation. Thecontent of β-hydroxy propionic acid in the acrylate (II) for comparisonbased on the solids thereof was found to be 4,200 ppm.

Production 3

[0161] An acrylate (III) for comparison was obtained by following theprocedure of Production 1, except that a commercially available acrylicacid (guaranteed reagent produced by Wako Junyaku K. K. ) was used forneutralization without being refined in advance by distillation. Thecontent of β-hydroxy propionic acid in the acrylate (III) for comparisonbased on the solids thereof was found to be 8,200 ppm.

Control Productions 4 and 5

[0162] Acrylates (IV) and (V) for comparison were obtained by followingthe procedure of Production 1, except that samples of acrylic acid whichhad been stored respectively for 120 hours and 240 hours after therefinement by distillation were used instead for neutralization. Thecontents of β-hydroxy propionic acid in the acrylates (IV) and (V) forcomparison based on the solids thereof were found to be respectively1,100 ppm and 1,900 ppm.

Control Production 6

[0163] An acrylate (VI) for comparison was obtained by following theprocedure of Production 1, except that acrylic acid which had beenstored at a temperature of 40° C. for 48 hours after the refinement bydistillation was used instead for neutralization. The content ofβ-hydroxy propionic acid in the acrylate (VI) for comparison based onthe solids thereof was found to be 1300 ppm.

Control Productions 7 and 8

[0164] Acrylates (VII) and (VIII) for comparison were obtained byfollowing the procedure of Production 8, except that samples of acrylicacid which had been stored respectively for 120 hours and 240 hoursafter the refinement by distillation were used instead forneutralization. The contents of β-hydroxy propionic acid in theacrylates (VII) and (VIII) for comparison based on the solids thereofwere found to be respectively 1,300 ppm and 2,100 ppm.

Control Productions 9 and 10

[0165] Acrylates (IX) and (X) for comparison were obtained by followingthe procedure of Production 11, except that aqueous acrylic acidsolutions which had been stored for 240 hours and 480 hours respectivelyafter the refinement by distillation were used instead forneutralization. The contents of β-hydroxy propionic acid in theacrylates (IX) and (X) based on the solids thereof were found to berespectively 3,300 ppm and 6,700 ppm.

Control Production 11

[0166] When the procedure of Production 8 was repeated, except that thetemperature of the neutralization system was elevated from a level inthe range of 20° to 40° C. to a level in the range of 50° to 60° C. forthe purpose of decreasing the time required for neutralization, the timeof neutralization was decreased from 120 minutes to 40 minutes. Thecontent of β-hydroxy propionic acid in the resultant acrylate was 2,100ppm. Hereinafter, this acrylate will be referred to as “acrylate (XI)for comparison.”

Control Production 12

[0167] An acrylate (XII) for comparison having a β-hydroxy propionicacid content of 1,800 ppm and a ratio of neutralization of 50 mol % wasobtained by following the procedure of Production 13, except that thesame acrylic acid as used in Control Production 1 was used forneutralization.

Control Production 13

[0168] An acrylate (XIII) for comparison having a β-hydroxy propionicacid content of 3,200 ppm and a ratio of neutralization of 100 mol % wasobtained by following the procedure of Production 15, except that thesame acrylic acid as used in Control Production 1 was used forneutralization.

EXAMPLE 1

[0169] A water-soluble unsaturated monomer (I) (concentration 37% andratio of neutralization 75 mol %) having a β-hydroxy propionic acidcontent of 40 ppm was obtained by dissolving 1.8 g of N,N′-methylenebis-acrylamide as a cross-linking agent in 5,500 g of the acrylate (I)obtained in Production 1.

[0170] The water-soluble unsaturated monomer deaerated with nitrogen gasfor 30 minutes was supplied to a reaction vessel constructed by fittinga lid on a jacketed twin-arm type kneader of stainless steel having aninner volume of 10 liters and provided with two sigma type vanes. Withthe water-soluble unsaturated monomer (I) kept at a constant temperatureof 30° C., the entrapped gas in the reaction system was displaced withnitrogen. Then, the reaction vessel was kept heated by passing hot waterat 35° C. through the jacket and in the meanwhile 0.3 mol % of ammoniumpersulfate and 0.03 mol % of sodium hydrogen sulfite were added to thereaction vessel. Two hours had elapsed between the time thewater-soluble unsaturated monomer (I) was prepared by the proceduredescribed above and the time polymerization of this monomer wasinitiated by the addition of an initiator.

[0171] The polymerization was initiated one minute after the addition ofthe initiator. The peak temperature in the reaction system reached 83°C. after 16 minutes thence. At this time, the resultant hydrogel polymerwas divided into particles about 5 mm in diameter. The stirring of theinterior was further continued. After 60 minutes following the start ofpolymerization, the hydrogel polymer was removed from the reactionvessel. The minute particles of the hydrogel polymer thus obtained werespread on a metallic net of 50 mesh and dried with hot air thereon at130° C. for 90 minutes. The dried particles were pulverized by the useof a vibration mill and further classified with a 20-mesh screen toobtain an absorbent resin of acrylate (1). The absorbent resin ofacrylate (1) was tested for residual monomer content, latent residualmonomer content, and absorption ratio. The results are shown in Table 1.

EXAMPLES 2 to 4

[0172] Absorbent resins of acrylate (2) to (4) were obtained byrepeating the procedure of Example 1, except that the time between thepreparation of the water-soluble unsaturated monomer (I) and the startof polymerization was increased to 6 hours, 12 hours, and 24 hoursrespectively by standing for necessary time. The physical properties ofthese absorbent resins are shown in Table 1.

EXAMPLES 5 to 7

[0173] Water-soluble unsaturated monomers (II) to (IV) having β-hydroxypropionic acid contents of 90 ppm, 190 ppm, and 100 ppm respectivelywere obtained by repeating the procedure of Example 1, except that theacrylates (II) to (IV) were used in the place of the acrylate (I).

[0174] By setting the water-soluble unsaturated monomers (II) to (IV)undergoing polymerization in the same manner as in Example 1, 2 hoursafter their preparation, absorbent resins of acrylate (5) to (7) wereobtained. The physical properties of these absorbent resins are shown inTable 1.

EXAMPLE 8

[0175] A water-soluble unsaturated monomer (V) having a β-hydroxypropionic acid content of 50 ppm was obtained by repeating the procedureof Example 1, except that the acrylate (V) was used instead of acrylate(I) in the preparation of water-soluble unsaturated monomer and 2.2 g ofpolyethylene glycol diacrylate (0.02 mol % based on the monomer)(average n number 8) was used in the place of the N,N′-methylenebis-acrylamide as the cross-linking agent.

[0176] The water-soluble unsaturated monomer (V) was polymerized in thesame manner as in Example 1, 6 hours after preparation. The gel polymerthus obtained was dried at 150° C. for 75 minutes and then pulverizedand classified in the same manner as in Example 1 to obtain an absorbentresin of acrylate (8). The physical properties of the absorbent resinare shown in Table 1.

EXAMPLES 9 and 10

[0177] Water-soluble unsaturated monomers (VI) and (VII) havingβ-hydroxy propionic acid contents of 80 ppm and 130 ppm respectivelywere obtained by following the procedure of Example 8, except that theacrylates (VI) and (VII) were used instead acrylate (V) in thepreparation of the water-soluble unsaturated monomer.

[0178] Then, the water-soluble unsaturated monomers (VI) and (VII) weretreated in the same manner as in Example 8 to obtain absorbent resins ofacrylate (9) and (10). The physical properties of these absorbent resinsare shown in Table 1.

EXAMPLE 11

[0179] A water-soluble unsaturated monomer (VIII) having a β-hydroxypropionic acid content of 230 ppm was obtained by following theprocedure of Example 1, except that the acrylate (VIII) was used insteadof the acrylate (I) in the preparation of the water-soluble unsaturatedmonomer and 13.6 g of trimethylol propane triacrylate (0.2 mol % basedon the monomer) was used instead of the N,N′-methylene bis-acrylamide asthe cross-linking agent.

[0180] The water-soluble unsaturated monomer (VIII) was polymerized inthe same manner as in Example 1, 2 hours after the preparation thereof.The gel-polymer consequently obtained was dried at 180° C. for 60minutes and then treated in the same manner as in Example 1 to obtain anabsorbent resin of acrylate (11). The physical properties of this resinare shown in Table 1.

EXAMPLES 12 and 13

[0181] Water-soluble unsaturated monomers (IX) and (X) having β-hydroxypropionic acid contents of 290 ppm and 390 ppm respec-tively wereobtained by following the procedure of Example 11, except that theacrylates (IX) and (X) were used instead of acrylate (VIII) in thepreparation of water-soluble unsaturated monomer.

[0182] Then, the water-soluble unsaturated monomers (IX) and (X) weretreated in the same manner as in Example 11 to obtain absorbent resinsof acrylate (12) and (13). The physical properties of these absorbentresins are shown in Table 1.

EXAMPLE 14

[0183] In a reaction vessel provided with a stirrer, a nitrogen inlettube, and a thermometer, a water-soluble unsaturated monomer (XI) havinga β-hydroxy propionic acid content of 230 ppm was prepared by dissolving30 g of corn starch in 600 g of water and further dissolving 718 g (3mols) of the acrylate (XI), 71 g (1 mol) of acrylamide, and 0.12 g (0.01mol % based on the monomer) of trimethylol propane triacrylate in theaqueous starch solution.

[0184] The water-soluble unsaturated monomer (XI) was left standing at30° C. for two hours. Then, it was blown with a forced current ofnitrogen gas for one hour to expel the dissolved oxygen. Thiswater-soluble unsaturated monomer was combined with 0.1 mol % of sodiumpersulfate as a polymerization catalyst and 0.05 mol % of 1-ascorbicacid and the resultant hydrogel was polymerized for 3 hours. Further,this hydrogel was dried in the form of a thick film by the use of adouble drum drier having a surface temperature of 150° C. The dry filmwas pulverized and classified with a 20-mesh screen to obtain anabsorbent resin of acrylate (14). The results of the analysis of thisabsorbent resin are shown in Table 1.

EXAMPLE 15

[0185] A water-soluble unsaturated monomer (XII) having a β-hydroxypropionic acid content of 70 ppm was obtained by dissolving 3.4 g (0.1mol % based on the monomer) of tetraethylene glycol diacrylate in 1,000g of acrylate (XII).

[0186] The water-soluble unsaturated monomer (XII) freshly prepared wasleft standing at 50° C. and blown with a forced current of nitrogen gasto expel the dissolved oxygen. Then, the monomer was spread in the formof a layer, 5 mm in thickness under an atmosphere of nitrogen and waspolymerized thereon by having 0.2 mol % of2,2′-azo-bis(2-amidinopropane) dihydrochloride sprayed thereon. Twohours had elapsed between the time the water-soluble unsaturated monomerwas prepared and the time the polymerization thereof was initiated.

[0187] The polymerization was immediately initiated. The gel polymerconsequently formed was removed from the reaction vessel after 10minutes of the polymerization. It was pulverized and dried with hot airat 150° C. for 60 minutes. The dry gel polymer was pulverized andclassified in the same manner as in Example 1 to obtain an absorbentresin of acrylate (15). The results of the analysis of the absorbentresin are shown in Table 1.

EXAMPLES 16 and 17

[0188] Absorbent resins of acrylate (16) and (17) were obtained byfollowing the procedure of Example 15, except that the water-solubleunsaturated monomer (XII) was left standing and polymerizing at 30° C.for 12 hours and 24 hours respectively after preparation untilpolymerization. The results of the analysis of these absorbent resinsare shown in Table 1.

EXAMPLE 18

[0189] A water-soluble unsaturated monomer of a concentration of 35%having a ratio of neutralization of 75% and a β-hydroxy propionic acidcontent of 70 ppm was prepared by using 83.4 g of the acrylate (XII),0.004 g of N,N′-methylene bis-acrylamide (0.0065 mol % based on themonomer) as a cross-linking agent, and 17.77 g of deionized water. Threehours after the completion of preparation, the water-soluble unsaturatedmonomer was blown with a forced current of nitrogen gas to expel thedissolved oxygen.

[0190] Separately, in a four-neck separable flask having an inner volumeof 500 ml and provided with a stirrer, a reflux condenser, athermometer, a nitrogen gas inlet tube, and a dropping funnel, 250 ml ofcyclohexane was placed, 2.0 g of sorbitan monostearate (HLB 4.7) as adispersant was. dissolved in the cyclohexane, and the resultant solutionwas blown with a forced current of nitrogen gas to expel the dissolvedoxygen.

[0191] The water-soluble unsaturated monomer was deaerated for one hourand 0.06 mol % of potassium persulfate was dissolved in the monomer. Theresultant solution was placed in the separable flask mentioned above andstirred to effect dispersion therein at a rate of 250 rpm. Then, theresultant mixture was heated to 60° C. to initiate a polymerizationreaction. (Incidentally, four hours had elapsed between the time thewater-soluble unsaturated monomer was prepared and the timepolymerization was initiated.)

[0192] After the start of polymerization, the polymerization mixture waskept at the temperature mentioned above for two hours and subsequentlysubjected to azeotropic dehydration. When the water content of theresultant polymer fell below 10%, the polymer was separated byfiltration and dried in an oven at 130° C. for 1 hour to obtain anabsorbent resin of acrylate (18). The physical properties of this resinare shown in Table 1.

EXAMPLE 19

[0193] A water-soluble unsaturated monomer (XIII) of a concentration of45% having a β-hydroxy propionic acid of 30 ppm and a ratio ofneutralization of 50% was obtained by following the procedure of Example1, except that the acrylate (XIII) was used instead of the acrylate (I)in the preparation of water-soluble unsaturated monomer.

[0194] The water-soluble unsaturated monomer (XIII) was set polymerizingin the same manner as in Example 1 four hours after preparation. Theresultant polymer was treated in the same manner as in Example 1 toobtain an absorbent resin of acrylate (19). The physical properties ofthis absorbent resin are shown in Table 1.

EXAMPLE 20

[0195] A water-soluble unsaturated monomer (XIV) of a concentration of34% having a β-hydroxy propionic acid content of 50 ppm and a ratio ofneutralization of 90% was obtained by following the procedure of Example1, except that the acrylate (XIV) was used instead of the acrylate (I)in the preparation of water-soluble unsaturated monomer.

[0196] The water-soluble unsaturated monomer (XIV) was set polymerizingin the same manner as in Example 1 24 hours after preparation and thentreated in the same manner as in Example 1 to obtain an absorbent resinof acrylate (20). The physical properties of this resin are shown inTable 1.

EXAMPLE 21

[0197] In a switching cast polymerization device made of stainless steel(SUS 316), lined with ethylene tetrafluoride resin, and having an innervolume of 300 mm×300 mm×50 mm, 4,000 g of a water-soluble unsaturatedmonomer (XV) having a β-hydroxy propionic acid content of 290 ppm wasplaced in its unmodified form as an acrylate (XV) and the entrapped gasin the device was displaced with nitrogen. The device containing theacrylate (XV) was immersed in a water bath at 30° C.

[0198] After the elapse of 24 hours following the completion of thepreparation of the acrylate, 0.05 mol % of ammonium persulfate and 0.02mol % of sodium hydrogen sulfite were added to the acrylate to effectpolymerization thereof. Five hours after the start of thepolymerization, the hydrogel polymer consequently formed was removedfrom the cast polymerization device. The polymer was out into cords byusing a meat chopper and then dried and pulverized in the same manner asin Example 1 to obtain a water-soluble resin of acrylate (20). Theresults of the analysis of this water-soluble resin (20) are shown inTable 1.

EXAMPLE 22

[0199] One hundred (100) parts of the absorbent resin (1) obtained inExample 1 was mixed with 1 part of glycerol, 2 parts of water, and 2parts of ethyl alcohol. The resultant mixture was heated at 190° C. for20 minutes. The absorbent resin (22) consequently obtained was testedfor absorption capacity, residual acrylic acid content, suction power,presence or absence of the formation of wetted clusters of powder, andincrease of residual monomer content at surface cross-linking. Theresults of the test are shown in Table 3.

EXAMPLE 23

[0200] An absorbent resin (23) was obtained by mixing 100 parts byweight of the absorbent resin (5) with 0.1 part of ethylene glycoldiglycidyl ether, 5 parts of water, and 1 part of isopropyl alcohol andheating the resultant mixture at 180° C. for 30 minutes. The results ofthe analysis of this absorbent resin (23) are shown in Table 3.

EXAMPLE 24

[0201] An absorbent resin (24) was obtained by mixing 100 parts of theabsorbent resin (6) with 0.5 part of diethylene glycol, 2 parts ofwater, and 1 part of isopropyl alcohol and heating the resultant mixtureat 150° C. for three hours. The results of the analysis of thisabsorbent resin (24) are shown in Table 3.

EXAMPLE 25

[0202] An absorbent resin (25) was obtained by dispersing 100 parts byweight of the absorbent resin (8) in a mixed solvent containing 300parts of methanol and 30 parts of water, mixing the resultant dispersionwith 0.1 parts of ethylene glycol diglycidyl ether, and heating theresultant dispersed mixture at 160° C. for 1 hour to effect evaporationto dryn. The results of the analysis of this resin (25) are shown inTable 3.

EXAMPLE 26

[0203] An absorbent resin (26) was obtained by adding 2.5 parts ofethylene carbonate, 2.5 parts of water, and 2.5 parts of acetone to 100parts of the absorbent resin (9) and heating the resultant mixture at180° C. for 1 hour. The results of the analysis of this absorbent resin(26) are shown in Table 3.

EXAMPLE 27

[0204] An absorbent resin (27) was obtained by mixing 100 parts byweight of the absorbent resin (11) with 1 part of finely divided silicondioxide powder (Aerosil) and further with a treating solution containing0.1 parts of ethylene glycol diglycidyl ether and 10 parts of water andheating the resultant mixture at 150° C. for 1 hour. The results of theanalysis of this absorbent resin (27) are shown in Table 3.

EXAMPLE 28

[0205] An absorbent resin (28) was obtained by mixing 100 parts byweight of the absorbent resin (12) with 1 part of aluminum sulfate, 1part of glycerin, and 8 parts of water and heating the resultant mixtureat 180° C. for 30 minutes. The results of the analysis of the absorbentresin (28) are shown in Table 3.

EXAMPLE 29

[0206] The absorbent resin (18) was dispersed in 250 ml of cyclo-hexane.Separately, a dispersion of ethylene glycol diglycidyl ether wasobtained in a flask by dissolving 0.5 g of sorbitan monolaurate(HLB=8.6, produced by Kao Soap Co., Ltd. and marketed under trademarkdesignation of “Reodol SP-10”) as a surfactant in 50 g of cyclohexaneand adding an aqueous solution of 0.04 g of ethylene glycol diglycidylether in 2 ml of water with vigorous agitation to the dispersion. Thedroplets in this dispersion had an average particle diameter of 3microns. An absorbent resin (29) was obtained by mixing this dispersionin a stirred state with a suspension of the absorbent resin (18),keeping the temperature of the system at 75° C. for 3 hours, separatingthe produced polymer by filtration, and drying the separated polymerunder a vacuum.

Controls 1 to 3

[0207] Water-soluble unsaturated monomers (I) to (III) having β-hydroxypropionic acid contents respectively of 2,500 ppm, 4,200 ppm, and 8,200ppm were obtained by following the procedure of Example 1, except thatthe acrylates (I) to (III) for comparison were used in the place ofacrylate (I) in the preparation of the water-soluble unsaturatedmonomer. The water-soluble unsaturated monomers (I) to (III) forcomparison were treated in the same manner as in Example 1 to produceabsorbent resin of acrylate (1) to (3). The physical properties of theseabsorbent resins are shown in Table 2.

Controls 4 to 6

[0208] Absorbent resins of acrylate (4) to (6) for comparison wereobtained by repeating the procedure of Control 1, except that the timeintervening between the preparation of the water-soluble monomer (I) forcomparison and the initiation of polymerization was increasedrespectively to 12 hours, 24 hours, and 240 hours. The physicalproperties of these absorbent resins (4) to (6) for comparison are shownin Table 2.

Controls 7 to 9

[0209] Water-soluble unsaturated monomers (IV) to (VI) for comparisonhaving β-hydroxy propionic acid contents respectively of 1,100 ppm,1,900 ppm, and 1,300 ppm were obtained by repeating the procedure ofExample 8, except that the acrylates (IV) to (VI) for comparison wereused in the place of acrylate (V). in the preparation of thewater-soluble unsaturated monomer. The water-soluble unsaturatedmonomers (IV) to (VI) for comparison were treated in the same manner asin Example 8 to obtain absorbent resins of acrylate (7) to (9) forcomparison. The physical properties of these absorbent resins are shownin Table 2.

Controls 10 to 12

[0210] Water-soluble unsaturated monomers (VII) to (IX) for comparisonhaving β-hydroxy propionic acid contents respectively of 1,300 ppm,2,100 ppm, and 3,300 ppm were obtained by repeating the procedure ofExample 8, except that the acrylates (VII) to (IX) for comparison wereused in instead of the acrylate (VIII) in the preparation ofwater-soluble unsaturated monomer. The water-soluble unsaturatedmonomers (VII) to (IX) for comparison were treated in the same manner asin Example 11 to obtain absorbent resins of acrylate (10) to (12) forcomparison. The physical properties of these absorbent resins are shownin Table 2.

Control 13

[0211] A water-soluble unsaturated monomer (X) for comparison having aβ-hydroxy propionic acid content of 6,700 ppm was obtained by repeatingthe procedure of Example 14, except that the acrylate (X) for comparisonwas used instead of the acrylate (XI) in the preparation ofwater-soluble unsaturated monomer. The water-soluble unsaturated monomer(X) for comparison was treated in the same manner as in Example 14 toobtain an absorbent resin of acrylate (13) for comparison. The physicalproperties of this absorbent resin are shown in Table 2.

Control 14

[0212] A water-soluble unsaturated monomer (XI) for comparison having aβ-hydroxy propionic acid content of 2,100 ppm was obtained by repeatingthe procedure of Example 15, except that the acrylate (XI) forcomparison was used instead of acrylate (XI) in the preparation of thewater-soluble unsaturated monomer. The water-soluble unsaturated monomer(XI) for comparison was treated in the same manner as in Example 15 toobtain an absorbent resin of acrylate (14) for comparison. The physicalproperties of this absorbent resin are shown in Table 2.

Controls 15 and 16

[0213] Absorbent resins of acrylate for comparison (15) and (16) wereobtained by repeating the procedure of Control 14, except that thewater-soluble unsaturated monomer (XI) prepared in Control 14 was storedat 30° C. respectively for 24 hours and 120 hours before polymerization.The physical properties of these absorbent resins are shown in Table 2.

Control 17

[0214] A water-soluble unsaturated monomer (XII) for comparison having aβ-hydroxy propionic acid content of 1,800 ppm was obtained by repeatingthe procedure of Example 19, except that acrylate (XII) for comparisonwas used instead of acrylate (XIII) in the preparation of water-solubleunsaturated monomer. The water-soluble unsaturated monomer (XII) wastreated in the same manner as in Example 19 to obtain an absorbent resinof acrylate (17) for comparison. The physical properties of thisabsorbent resin are shown in Table 2.

Control 18

[0215] A water-soluble unsaturated monomer (XIII) for comparison havinga β-hydroxy propionic acid content of 3,200 ppm was obtained byfollowing the procedure of Example 21, except that the acrylate (XIII)for comparison was used instead of the acrylate (XV) in the preparationof water-soluble unsaturated monomer. The water-soluble unsaturatedmonomer (XIII) was treated in the same manner as in Example 21 to obtainan absorbent resin of acrylate (28) for comparison. The physicalproperties of this absorbent resin are shown in Table 2.

Control 19

[0216] A water-soluble unsaturated monomer (XIV) for comparison having aβ-hydroxy propionic acid content of 2,800 ppm was obtained by repeatingthe procedure of Example 21, except that 2,590 ppm of β-hydroxypropionic acid was separately added to acrylate (XV). The water-solubleunsaturated monomer (XIV) was treated in the same manner as in Example21 to obtain an absorbent resin of acrylate (19) for comparison. Thephysical properties of this absorbent resin are shown in Table 2.

Controls 20 to 22

[0217] Absorbent resins (20) to (22) for comparison were obtained byrepeating the procedures of Examples 22 to 24, except that absorbentresins (1), (5) and (6) for comparison were used instead of absorbentresins (1) to (3) as absorbent resins for cross-linking surface regionsthereof. The physical properties of these absorbent resins are shown inTable 2.

Controls 23 and 24

[0218] Absorbent resins (23) and (24) for comparison were obtained byrepeating the procedures of Examples 25 and 26, except that an absorbentresins (7) and (8) for comparison were used instead of absorbent resins(8) and (9) for cross-linking the surface regions. The physicalproperties of these absorbent resins are shown in Table 3.

Controls 25 and 26

[0219] Absorbent resins (25) and (26) for comparison were obtained byrepeating the procedures of Examples 27 and 28, except that absorbentresins (11) and (12) for comparison were used instead of absorbentresins (11) and (12) in cross-linking surface regions. The physicalproperties of these absorbent resins are shown in Table 3.

[0220] Analyzed results of acrylic acid salt polymers obtained inExamples 1 to 21 and Controls 1-19 are shown in Tables 1 and 2 andanalyzed results of acrylic acid salt polymers wherein surface region iscross-linked obtained in Examples 22 to 29 and Controls 20 to 26 areshown in Table 3.

[0221] Further, the acrylic acid salt polymers obtained from thewater-soluble unsaturated monomer containing not more than 100 ppm ofβ-hydroxy propionic acid in Examples contained not more than 100 ppm ofβ-hydroexy propionic acid, while the polymer obtained in Controlscontained not less than 1000 ppm of β-hydroexy propionic acid. TABLE 1β-hydroxy Time of propionic acid standing after Latent Water-solublecontent in monomer Absorption Residual residual Resin unsaturatedmonomer preparation capacity monomer monomer obtained monomer used (ppm)(hr) (g/g) (ppm) (ppm) Monomer Example 1 Absorbent resin (1) (I) 40 2 5135 max. 5 Example 2 Absorbent resin (2) ″ 40 6 51 55 ″ Example 3Absorbent resin (3) ″ 40 12 51 80 ″ Example 4 Absorbent resin (4) ″ 4024 50 130 ″ Example 5 Absorbent resin (5) (II) 90 2 51 45 ″ Example 6Absorbent resin (6) (III) 190 2 51 60 15 Example 7 Absorbent resin (7)(IV) 100 2 51 45 10 Example 8 Absorbent resin (8) (V) 50 6 64 80 max. 5Example 9 Absorbent resin (9) (VI) 80 6 64 90 10 Example 10 Absorbentresin (10) (VII) 130 6 64 110 10 Example 11 Absorbent resin (11) (VIII)230 2 40 140 30 Example 12 Absorbent resin (12) (IX) 290 2 40 150 30Example 13 Absorbent resin (13) (X) 390 2 40 160 40 Example 14 Absorbentresin (14) (XI) 230 3 70 220 20 Example 15 Absorbent resin (15) (XII) 702 43 220 max. 5 Example 16 Absorbent resin (16) ″ 70 12 43 260 ″ Example17 Absorbent resin (17) ″ 70 24 43 300 ″ Example 18 Absorbent resin (18)(XII′) 70 2 75 60 ″ Example 19 Absorbent resin (19) (XIII) 30 4 42 40 ″Example 20 Absorbent resin (20) (XIV) 50 24 52 50 ″ Example 21water-soluble resin (XV) 290 24 - 300 30 (21)

[0222] TABLE 2 β-hydroxy Time of propionic acid standing after LatentWater-soluble content in monomer Absorption Residual residual Resinunsaturated monomer preparation capacity monomer monomer obtainedmonomer used (ppm) (hr) (g/g) (ppm) (ppm) Monomer Control 1 Absorbentresin (1) (I) 2500 2 49 420 420 Control 2 Absorbent resin (2) (II) 42002 48 700 870 Control 3 Absorbent resin (3) (III) 8200 2 48 870 1200Control 4 Absorbent resin (4) (I) 2500 12 49 470 450 Control 5 Absorbentresin (5) ″ 2500 24 49 560 460 Control 6 Absorbent resin (6) ″ 2500 24047 1560 480 Control 7 Absorbent resin (7) (IV) 1100 6 63 300 140 Control8 Absorbent resin (8) (V) 1900 6 63 440 250 Control 9 Absorbent resin(9) (VI) 1300 6 63 340 130 Control 10 Absorbent resin (10) (VII) 1300 239 490 140 Control 11 Absorbent resin (11) (VIII) 2100 2 38 550 240Control 12 Absorbent resin (12) (IX) 3300 2 38 730 410 Control 13Absorbent resin (13) (X) 6700 3 67 420 900 Control 14 Absorbent resin(14) (XI) 2100 2 42 420 240 Control 15 Absorbent resin (15) ″ 2100 24 42830 320 Control 16 Absorbent resin (16) ″ 2100 120 41 930 330 Control 17Absorbent resin (17) (XII) 1800 4 40 1300 340 Control 18 Water-solubleresin (XIII) 3200 24 - 600 450 (18) Control 19 Water-soluble resin (XIV)2800 24 - 670 510 (19)

[0223] TABLE 3 Absorbent β-hydroxy Increment to Increment to resinpropionic residual monomer residual monomer subjected to acid content byby heating at Absorbent surface content in Absorption Residual Suctionsurface cross- 180° C. for three resin cross- monomer capacity monomerpower linking hours obtained linking (ppm) (g/g) (ppm) (g/g) (ppm) (ppm)Absorbent resin Example 22 (22) (1) 40 52 35 17.3    5> max. 5 Example23 (23) (5) 90 53 45 17.2    5> ″ Example 24 (24) (6) 190 51 65 17.0  5″ Example 25 (25) (8) 50 66 80 16.8    5> max. 5 Example 26 (26) (9) 8064 90 16.7 ″ ″ Example 27 (27) (11) 230 41 150 16.2  10 30 Example 28(28) (12) 290 42 160 16.2  10 30 Example 29 (29) (18) 70 77 60 16.0   5> 5 Absorbent resin for control for control Control 20 (20) (1) 250049 460 16.8  40 400 Control 21 (21) (2) 4200 49 800 16.7 100 820 Control22 (22) (3) 8200 48 1080 16.5 210 1000 Control 23 (23) (7) 1100 63 39016.3  90 120 Control 24 (24) (8) 1900 62 560 16.5 120 210 Control 25(25) (11) 2100 38 660 15.7 110 190 Control 26 (26) (12) 3300 39 980 15.9250 250

Production 16

[0224] A sample of acrylic acid obtained from the site of acrylic acidproduction at Himeji Plant of Nippon Shokubai Kagaku Kogyo Co., Ltd. wasrefined by distillation. The acrylic acid fresh from the distillationwas stored at 30° C. for two hours and then neutralized by the proceduredisclosed in EP-A-0372706.

[0225] A distillation flask provided with a stirrer was charged with1,944 g of deionized water. With the temperature of the neutralizationreaction system in the flask kept at a level in the range of 20° C. to40° C., 1,390 g of acrylic acid and 1,480 g of an aqueous 48 wt % sodiumhydroxide solution were added simultaneously at a sodiumhydroxide/acrylic acid dropping ratio in the range of 0.9 to 0.95 over aperiod of 100 minutes into the flask. After the completion of thedropwise addition, 160 g of an aqueous 48 wt % sodium hydroxide solutionwas supplied to adjust the ratio of neutralization of the neutralizationreaction system in the flask to 102 mol %. Then, the temperature of theneutralization reaction system was adjusted to 40° C. and the product ofneutralization was left to age for 30 minutes. After the completion ofaging, 28 g of acrylic acid was supplied over a period of 1 minute tothe neutralization reaction system to obtain 5,002 g of an acrylate(XVI) of a concentration of 37% having a ratio of neutralization of 100mol %.

[0226] A monomer (1) of a concentration of 38% having a ratio ofneutralization of 75% (55% of sodium salt and 20% of ammonium salt) wasobtained by adding 774 g of acrylic acid aged for 2 hours afterdistillation and 1,063 g of deionized water to 3,338 g of acrylate (XVI)resulting from neutralization and adding 325 g of an aqueous 25% ammoniasolution thereto, and further adding thereto 2.83 g (0.04 mol % based onthe monomer) of trimethylol propane triacrylate as a cross-linkingagent. The monomer (1), on being analyzed by liquid chromatography, wasfound to have a β-hydroxy propionic acid content of 25 ppm.

Production 17

[0227] A monomer (2) of a concentration of 38% having a ratio ofneutralization of 75% (35% of sodium salt and 40% of ammonium salt) wasobtained by repeating the procedure for the production. of the acrylate(XVI) in the preparation of the monomer in Production 16, except that2,148 g of acrylate (XVI), 1,132 g of acrylic acid, 1,563 g of deionizedwater, 657 g of an aqueous 25% ammonia solution, and 2.86 g (0.04 mol %based on the monomer) of trimethylol propane triacrylate were usedinstead. The β-hydroxy propionic acid content in the monomer (2) wasfound to be 18 ppm.

Production 18

[0228] A monomer (3) of a concentration of 38% having a ratio ofneutralization of 75% (65% of sodium salt and 10% of ammonium salt) wasobtained by following the procedure for the production of the acrylate(XVI) in the preparation of the monomer in Production 16, except that3,922 g of acrylate (XVI), 599 g of acrylic acid, 817 g of deionizedwater, 162 g of an aqueous 25% ammonia solution, and 2.81 g (0.04 mol %based on the monomer) of trimethylol propane triacrylate were usedinstead. The β-hydroxy propionic acid content in the monomer (3) wasfound to be 22 ppm.

Production 19

[0229] An acrylate (XVII) was obtained by following the procedure ofProduction 16, except that acrylic acid aged for 12 hours after beingrefined by distillation was used instead of the refined acid aged for 2hours in the preparation of the acrylate and monomer. A monomer (4) of aconcentration of 38% having a ratio of neutralization of 75% (55% ofsodium salt and 20% of ammonium salt) was obtained by repeating theprocedure of Production 1 using the acrylate (XVII) instead. Theβ-hydroxy propionic acid content in the monomer (4) was found to be 50ppm.

Production 20

[0230] An acrylate (XVIII) was obtained by following the procedure ofProduction 16, except that acrylic acid aged for 24 hours after beingrefined by distillation was used instead of acrylic acid aged for twohours after distillation in the preparation of the acrylate and monomer.A monomer (5) of a concentration of 38% having a ratio of neutralizationof 75% (55% of sodium salt and 20% of ammonium salt) was obtained byfollowing the procedure of Production 1 using acrylate (XVIII) instead.The β-hydroxy propionic acid content in the monomer (5) was found to be110 ppm.

Production 21

[0231] A commercially available acrylic acid (guaranteed reagentproduced by Wako Junyaku K. K. ) was refined by distillation. Therefined acrylic acid was converted into an aqueous 80% solution having ahigh point of solidification thereby allowing easy handling. Thesolution was stored at a temperature of 30° C. for two hours and thenneutralized by the following procedure.

[0232] A distillation flask provided with a stirrer was charged with2,272 g of deionized water and 1,640 g of an aqueous 48 wt % sodiumhydroxide solution. Then, with the temperature of the neutralizationreaction system kept at 10° C., 2,361 g of an aqueous 80% acrylic acidsolution was supplied meanwhile over a period of 6 hours to the flask toobtain an acrylate (XIX) of a concentration of 37% having a ratio ofneutralization of 75 mol%.

[0233] Then, a monomer (6) of a concentration of 38% having a ratio ofneutralization of 75% (55% of sodium salt and 20% of ammonium salt) wasobtained by repeating the procedure of Production 16 using acrylate(XIX) instead. The β-hydroxy propionic acid content in the monomer (6)was found to be 50 ppm.

Production 22

[0234] A monomer (7) of a concentration of 38% having a ratio ofneutralization of 75% (75% of sodium salt) was obtained by repeating theprocedure of Production 16, except that 4,500 g of acrylate (XVI), 425 gof acrylic acid, 575 g of deionized water, and 3.39 g (0.03 mol % basedon the monomer) of polyethylene glycol diacrylate (average n=7) wereused in the same manner as the production of acrylate (XVI) in thepreparation of the monomer. The β-hydroxy propionic acid content in themonomer (7) was found to be 30 ppm.

Control Production 14

[0235] An acrylate (XIV) for comparison was obtained by repeating theprocedure of Production 16, except that acrylic acid aged for 200 hoursafter being refined by distillation was used instead of refined acrylicacid aged for 2 hours in the production of the acrylate and monomer inthe preparation of the monomer. A monomer (1) for comparison of aconcentration of 38% having a ratio of neutral-ization of 75% (55% ofsodium salt and 20% of ammonium salt) was obtained by -following theprocedure of Example 16 of Production using the acrylate (XIV) forcomparison instead. The β-hydroxy propionic acid content in the monomer(1) for comparison was found to be 1,100 ppm.

Production 15

[0236] An acrylate (XV) for comparison was obtained by repeating theprocedure of Production 16, except that a commercially available acrylicacid (guaranteed reagent produced by Wako Junyaku K. K. ) was used inits unmodified form instead of refined acrylic acid aged for two hoursin the production of the acrylate and monomer in the preparation of themonomer. A monomer (2) for comparison of a concentration of 38% having aratio of neutralization of 75% (55% of sodium salt and 20% of ammoniumsalt) was obtained by using the acrylate (XV) for comparison. Theβ-hydroxy propionic acid content in the monomer (2) for comparison wasfound to be 3,200 ppm.

Control Production 16

[0237] An acrylate (XVI) for comparison was obtained by following theprocedure of Production 21, except that the temperature of theneutralization reaction system was raised from 10° C. to a level in therange of 50° to 60° C. for the purpose of reducing the duration ofneutralization. The time required for the neutralization consequentlydecreased from six hours to 40 minutes. Then, a monomer (3) forcomparison of a concentration of 38% having a ratio of neutralization of75% (55% of sodium salt and 20% of ammonium salt) was obtained byfollowing the procedure of Production 16 using acrylate (XVI) instead.The β-hydroxy propionic acid content in the monomer (5) was found to be1,300 ppm.

Control Production 17

[0238] A monomer (4) for comparison was obtained by repeating theprocedure of Production 22, except that an aqueous acrylic acid solutionstored for 200 hours after being refined by distillation was used as anaqueous 80% acrylic acid solution for the neutralization. The β-hydroxypropionic acid content in the monomer (4) for comparison was found to be2,900 ppm.

Control Production 18

[0239] A monomer (5) for comparison having a β-hydroxy propionic acidcontent of 1,600 ppm was obtained- by repeating the procedure ofProduction 22, except that the same acrylic acid as used in ControlProduction 14 was used instead of the acrylic acid aged for two hoursafter distillation in the preparation of the monomer.

EXAMPLE 30

[0240] In a reaction vessel constructed by fitting a lid to a jacketedtwin-arm type kneader made of stainless steel, having an inner volume of10 liters, and provided with two sigma type vanes, 5,500 g of themonomer (1) obtained in Production 16 deaerated with nitrogen gas for 30minutes was placed. Then, with the reaction vessel kept heated bypassage of hot water at 35° C. through the jacket, 0.3 mol % of ammoniumpersulfate and 0.03 mol % of sodium hydrogen sulfite were added to thereaction vessel. Two hours had elapsed between the time the monomer wasprepared by the procedure described above and the time polymerizationwas initiated by the addition of an initiator. Polymerization wasinitiated 1 minute after the addition of the initiator. The gel polymerconsequently formed was finely divided into particles about 5 mm indiameter in 16 minutes. The stirring continued for a further 44 minutesbefore the gel polymer was removed from the reaction vessel.

[0241] The minute particles of the gel polymer consequently obtainedwere spread on a 50-mesh metallic net and then heat-treated thereon at180° C. for 60 minutes with the aid of hot air. The dry particles thusformed were pulverized with a vibration mill and then classified with a20-mesh screen to obtain an absorbent resin (30). The results are shownin Table 4.

EXAMPLES 31 and 32

[0242] Absorbent resins (31) and (32) were obtained by repeating theprocedure of Example 30, except that the time between the completion ofthe preparation of the monomer (a) and the introduction of the initiatorwas changed respectively to six hours and 24 hours. The results areshown in Table 4.

EXAMPLES 33 and 34

[0243] Absorbent resins (33) and (34) were obtained by repeating theprocedure of Example 30, except that the temperature for drying andheating the gel polymer resulting from polymerization was changedrespectively to 150° C. and 120° C. The results are shown in Table 4. Itis clearly noted from the data of Table 4 that the absorption ratio waslowered and the residual monomer content was increased by lowering thetemperature of drying.

EXAMPLES 35 to 38

[0244] Absorbent resins (35) to (38) were obtained by repeating theprocedure of Example 16, except that the monomers (2) to (5) were usedinstead of monomer (1) for polymerization. The results are shown inTable 4.

EXAMPLE 39

[0245] In a switching cast polymerization device made of stainless steel(SUS 316), lined with ethylene tetrafluoride resin, and having an innervolume of 300 mm×300 mm×50 mm, 4,000 g of monomer (6) deaerated withnitrogen was placed and the entrapped gas in the device was displacedwith nitrogen. The device containing the monomer was immersed in a waterbath kept at 30° C. Then, 0.05 mol % of ammonium persulfate and 0.02 mol% of sodium hydrogen sulfite were added to the device to initiatepolymerization. Four hours had elapsed between the time the monomer (6)was prepared by the procedure described above and the time thepolymerization was initiated.

[0246] After 5 hours from the start of the polymerization, the gelpolymer consequently formed was withdrawn from the cast polymerizationdevice. The gel polymer was then pulverized into particles about 5 mm indiameter by using a meat chopper and dried and heat-treated in the samemanner as in Example 1 to obtain an absorbent resin (39). The resultsare shown in Table 4.

EXAMPLE 40

[0247] The procedure of Example 30 was repeated to polymerize monomer(7) instead of monomer (1). The gel polymer which was formed within 30minutes of commencing the polymerization was combined with 160 g (10 mol% based on the monomer) of an aqueous 25% ammonia solution and subjectedto continued polymerization for 30 minutes. The resultant gel polymerhaving a ratio of neutralization of 85% (inclusive of 10 mol % ofammonium salt) was dried at 170° C. for 80 minutes with the aid of hotair and thereafter treated in the same manner as in Example 1 to obtainan absorbent resin (40). The results are shown in Table 4.

EXAMPLE 41

[0248] The procedure of Example 30 was repeated to polymerize a monomerformed of monomer (7) and 142 g (10 mol % based on the monomer) of ureaas an ammonia precursor instead of monomer (1). The resultant gelpolymer having a ratio of neutralization of 95% (inclusive of 20 mol %of ammonium salt) was dried at 190° C. for 60 minutes with the aid ofhot air and thereafter treated in the same manner as in Example 30 toobtain an absorbent resin (41). The results are shown in Table 4.

EXAMPLE 42

[0249] An absorbent resin (42) was obtained by mixing 100 parts ofabsorbent resin (30) powder obtained in Example 30 with 1 part ofglycerol, 2 parts of water, and 2 parts of ethyl alcohol and thenheat-treating the resultant mixture at 190° C. for 40 minutes. Theresults are shown in Table 5.

EXAMPLE 43

[0250] An absorbent resin (43) was obtained by mixing 100 parts ofabsorbent resin (35) powder with 2 parts of propylene glycol, 3 parts ofwater, and 2 parts of isopropyl alcohol and then heat-treating theresultant mixture at 150° C. for 60 minutes. The results are shown inTable 5.

EXAMPLE 44

[0251] An absorbent resin (44) was obtained by mixing 100 parts ofabsorbent resin (36) powder with 0.1 part of ethylene glycol diglycidylether, 5 parts of water, and 1 part of isopropyl alcohol and thenheat-treating the resultant mixture at 180° C. for 30 minutes. Theresults are shown in Table 5.

EXAMPLE 45

[0252] An absorbent resin (45) was obtained by mixing 100 parts ofabsorbent resin (37) powder with 0.1 part of ethylene glycol diglycidylether, 30 parts of methanol, and parts of water and then heat-treatingthe resultant mixture at 180° C. for one hour. The results are shown inTable 5.

EXAMPLE 46

[0253] An absorbent resin (46) was obtained by mixing 100 parts ofabsorbent resin (38) powder with 2.5 parts of ethylene carbonate, 2.5parts of water, and 2.5 parts of acetone and then heat-treating theresultant mixture at 230° C. for one hour. The results are shown inTable 5.

EXAMPLE 47

[0254] An absorbent resin (47) was obtained by mixing 100 parts ofabsorbent resin (39) powder with 0.1 part of ethylene glycol diglycidylether and 10 parts of water in the presence of 1 part of fine silicondioxide powder (Aerosil) and then heat-treating the resultant mixture at180° C. for one hour. The results are shown in Table 5.

EXAMPLE 48

[0255] An absorbent resin (48) was obtained by mixing 100 parts byweight of absorbent resin (4) powder with 1 part of aluminum sulfate, 1part of glycerol, and 8 parts of water, and heat-treating the resultantmixture at 180° C. for 30 minutes. The results are shown in Table 2.

Control 27

[0256] An absorbent resin (27) for comparison was obtained by repeatingthe procedure of Example 30, except that monomer (1) for comparison wasused instead of monomer (1). The results are shown in Table 4.

Control 28

[0257] An absorbent resin (28) for comparison was obtained by repeatingthe procedure of Control 27, except that the time between the completionof the preparation of monomer (1) for comparison and the introduction ofthe initiator was changed to 24 hours. The results are shown in Table 4.

Control 29

[0258] An absorbent resin (29) for comparison was obtained by repeatingthe procedure of Control 27, except that the temperature for drying andheat-treating the gel polymer resulting from polymerization was changedto 120° C. The results are shown in Table 1.

Controls 30 to 33

[0259] Absorbent resins (30) to (33) for comparison were obtained byrepeating the procedure of Example 30, except that monomers (2) to (5)for comparison were used respectively instead of monomer (1). Theresults are shown in Table 4.

Control 34

[0260] An absorbent resin (34) for comparison was obtained by repeatingthe procedure of Example 42, except that a powder of absorbent resin(27) for comparison was used instead of absorbent resin (30) powder forthe purpose of cross-linking the surface region. The results are shownin Table 5.

Controls 35 to 38

[0261] Absorbent resins (35) to (38) for comparison were obtained byrepeating the procedures of Examples 43 to 46 respectively, except thatpowders of absorbent resins (30) to (33) for comparison were usedinstead of absorbent resins (35) to (38) for the purpose ofcross-linking surface regions. The results are shown in Table 5.

[0262] It is clearly noted from the data of Table 4 and Table 5 that anabsorbent resin produced by the method according to this invention hasonly a small residual monomer content and allows neither generation norgrowth of residual monomer as a consequence of surface treatment or asubsequent heat treatment. Further, due to the heat treatment which isperformed at an elevated temperature, the produced absorbent resin has ahigh absorption capacity.

[0263] The acrylic acid salt polymers obtained from the water-solubleunsaturated-monomer containing not more than 100 ppm of β-hydroxypropionic acid in Examples contained not more than 100 ppm of β-hydroxypropionic acid, while the polymers obtained in Controls contained notless than 1000 ppm of β-hydroxy propionic acid. TABLE 4 β-hydroxy Timeof Increase or decrease propionic acid standing after TemperatureResidual inresidual monomer content in preparation of of heat Absorptionmonomer content by 3 hours Absorbent resin Monomer monomer monomertreatment capacity content heating at 180° C. obtained used (ppm) (hr)(° C.) (g/g) (ppm) (ppm) Example Absorbent resin Monomer ↑ 30 ↑ (30) (1)25 2 180 53 13 −5 ↑ 31 ↑ (31) ↑ ↑ 6 ↑ 53 15 −4 ↑ 32 ↑ (32) ↑ ↑ 24 ↑ 5223 −4 ↑ 33 ↑ (33) ↑ ↑ 2 150 51 26 −5 ↑ 34 ↑ (34) ↑ ↑ ↑ 120 44 55 −20↑ 35 ↑ (35) (2) 18 ↑ 180 53 5 −2 ↑ 36 ↑ (36) (3) 22 ↑ ↑ 53 20 ±0 ↑ 37↑ (37) (4) 50 ↑ ↑ 53 19 −7 ↑ 38 ↑ (38) (5) 110 ↑ ↑ 52 28 −10 ↑ 39 ↑ (39)(6) 50 4 ↑ 53 16 −5 ↑ 40 ↑ (40) (7) 30 2 170 56 10 ±0 ↑ 41 ↑ (41) ↑ ↑ ↑190 60 15 −3 Absorbent resin Monomer Control for Control for control   27    (27) (1) 1100 2 180 52 140 +20 ↑ 28 ↑ (28) ↑ ↑ 24 ↑ 51 150 +10↑ 29 ↑ (29) ↑ ↑ 2 120 43 320 +20 ↑ 30 ↑ (30) (2) 3200 ↑ 180 51 290 +40↑ 31 ↑ (31) (3) 1300 ↑ ↑ 52 160 +20 ↑ 32 ↑ (32) (4) 2900 ↑ ↑ 51 430 +50↑ 33 ↑ (33) (5) 1600 ↑ ↑ 52 520 +240

[0264] TABLE 5 β-hydroxy Increase or decrease Absorbent propionicinresidual monomer resin acid Temperature Residual content due toAbsorbent subjected to content in of heat Absorption Suction monomersurface cross- resin surface monomer treatment capacity power contentlinking obtained cross-linking (ppm) (° C.) (g/g) (g/g) (ppm) (ppm)Absorbent Absorbent Example resin resin ↑ 42 ↑ (42)    (30) 25 190 5117.3 9 −4 ↑ 43 ↑ (43) ↑ (35) 18 150 51 17.2 3 −2 ↑ 44 ↑ (44) ↑ (36) 22180 52 17.0 20 ±0 ↑ 45 ↑ (45) ↑ (37) 50 180 48 16.7 16 −3 ↑ 46 ↑ (46)↑ (38) 110 230 50 16.9 20 −8 ↑ 47 ↑ (47) ↑ (39) 50 180 51 16.5 14 −2↑ 48 ↑ (48) ↑ (40) 30 180 53 16.2 10 ±0 Absorbent resin for Controlcontrol for control    34    (34)    (27) 1100 190 49 16.8 150 +10 ↑ 35↑ (35) ↑ (30) 3200 150 49 16.9 310 +20 ↑ 36 ↑ (36) ↑ (31) 1300 180 5016.5 170 +10 ↑ 37 ↑ (37) ↑ (32) 2900 180 48 16.7 460 +30 ↑ 38 ↑ (38)↑ (33) 1600 230 49 16.8 580 +60

What is claimed is:
 1. A method for the production of a hydrophilicresin which comprises polymerizing a water-soluble unsaturated monomercontaining 50 to 100 mol % of an acrylate having a ratio ofneutralization in the range of 30 to 100 mol % and not more. than 1,000ppm of β-hydroxy propionic acid (salt).
 2. A method according to claim1, wherein said water-soluble unsaturated monomer contains 50 to 0 mol %of at least one unsaturated monomer selected from the group consistingof hydrophilic unsaturated monomers and hydrophobic unsaturatedmonomers.
 3. A method according to claim 2, wherein said water-solubleunsaturated monomer contains not more than 300 ppm of β-hydroxypropionic acid (salt).
 4. A method according to claim 1, wherein saidacrylate is at least one member selected from the group consisting ofalkali metal salts and ammonium salts.
 5. A method according to claim 4,wherein said alkali metal salt is at least one member selected from thegroup consisting of sodium salts and potassium salts.
 6. A methodaccording to claim 1, wherein said hydrophilic resin has a cross-linkedstructure.
 7. A method according to claim 6, wherein said acrylate is analkali metal salt and an ammonium salt and said absorbent polymer or adry product thereof has been subjected to a heat treatment.
 8. A methodaccording to claim 7, wherein an ammonium salt accounts for 10 to 40 mol% of the acid group in said absorbent resin.
 9. A method according toclaim 7, wherein the temperature of said heat treatment is in the rangeof 150° to 250° C.
 10. A method according to claim 4, wherein said heattreatment is carried out at the step of drying said absorbent resin. 11.A method for the production of a hydrophilic resin which comprisespolymerizing a water-soluble unsaturated monomer containing 50 to 100mol % of an acrylate having a ratio of neutralization in the range of 30to 100 mol % and not more than 1,000 ppm of β-hydroxy propionic acid(salt) and then cross-linking the surface region of the resultantacrylate polymer by treating said polymer with a second cross-linkingagent having at least two groups capable of reacting with the functionalgroup within the molecular unit of said polymer.
 12. A method accordingto claim 11, wherein said water-soluble unsaturated monomer contains 50to 0 mol % of at least one unsaturated monomer selected from the groupconsisting of hydrophilic unsaturated monomers and hydrophobicunsaturated monomers.
 13. A method according to claim 12, wherein saidwater-soluble unsaturated monomer contains not more than 300 ppm ofβ-hydroxy propionic acid (salt).
 14. A method according to claim 11,wherein said acrylate is at least one member selected from the groupconsisting of alkali metal salts and ammonium salts.
 15. A methodaccording to claim 14, wherein said alkali metal salt is at least onemember selected from -the group consisting of sodium salts and potassiumsalts.
 16. A method according to claim 11, wherein said hydrophilicresin has a cross-linked structure.
 17. A method according to claim 11,wherein said second cross-linking agent is at least one member selectedfrom the group consisting of polyhydric alcohols, polyepoxy compounds,polyamines, polyaziridines, polyaldehydes, polyisocyanates,polyoxazolines, alkylene carbonates, and polyvalent metals.
 18. A methodaccording to claim 16, wherein said acrylate is an alkali metal salt andan ammonium salt and said absorbent polymer or the dry product thereofhas undergone a heat treatment.
 19. A method according to claim 18,wherein an ammonium salt accounts for 10 to 40 mol % of the acid groupin said absorbent resin.
 20. A method according to claim 18, wherein thetemperature of said heat treatment is in the range of 1500 to 250° C.21. A method according to claim 18, wherein said heat treatment iscarried out at the step of drying said absorbent resin.
 22. An acrylicacid salt polymer composition which contains 1 to 1000 ppm of β-hydroxypropionic acid (salt) and not more than 100 ppm of a residual monomer.23. A composition according to claim 21, wherein neutralization ratio ofcarboxyl group is 30 to 100 mol%.
 24. A composition according claim 23,wherein said acrylic acid salt polymer is an absorbent resin.
 25. Acomposition according to claim 22, wherein a content of β-hydroxypropionic acid (salt) is 1 to 100 ppm.